diff --git a/compiler/erg_compiler/context/initialize/mod.rs b/compiler/erg_compiler/context/initialize/mod.rs
index e367056f..58072bd2 100644
--- a/compiler/erg_compiler/context/initialize/mod.rs
+++ b/compiler/erg_compiler/context/initialize/mod.rs
@@ -1794,6 +1794,7 @@ impl Context {
         let t_exit = t_quit.clone();
         let t_repr = nd_func(vec![kw("object", Obj)], None, Str);
         let t_round = nd_func(vec![kw("number", Float)], None, Int);
+        let t_str = nd_func(vec![kw("object", Obj)], None, Str);
         let t_unreachable = nd_func(vec![], None, Never);
         self.register_builtin_py_impl("abs", t_abs, Immutable, Private, Some("abs"));
         self.register_builtin_py_impl("ascii", t_ascii, Immutable, Private, Some("ascii"));
@@ -1855,6 +1856,7 @@ impl Context {
         self.register_builtin_py_impl("quit", t_quit, Immutable, Private, Some("quit"));
         self.register_builtin_py_impl("repr", t_repr, Immutable, Private, Some("repr"));
         self.register_builtin_py_impl("round", t_round, Immutable, Private, Some("round"));
+        self.register_builtin_py_impl("str", t_str, Immutable, Private, Some("str"));
         // TODO: original implementation
         self.register_builtin_py_impl(
             "unreachable",
diff --git a/compiler/erg_parser/lex.rs b/compiler/erg_parser/lex.rs
index 835bf95c..160d10d5 100644
--- a/compiler/erg_parser/lex.rs
+++ b/compiler/erg_parser/lex.rs
@@ -69,6 +69,19 @@ impl Runnable for LexerRunner {
     }
 }
 
+#[derive(Debug, PartialEq, Eq)]
+pub enum Interpolation {
+    SingleLine,
+    MultiLine,
+    Not,
+}
+
+impl Interpolation {
+    pub const fn is_in(&self) -> bool {
+        matches!(self, Self::SingleLine | Self::MultiLine)
+    }
+}
+
 /// Lexes a source code and iterates tokens.
 ///
 /// This can be used as an iterator or to generate a `TokenStream`.
@@ -85,6 +98,7 @@ pub struct Lexer /*<'a>*/ {
     lineno_token_starts: usize,
     /// 0-origin, indicates the column number in which the token appears
     col_token_starts: usize,
+    interpol_stack: Vec<Interpolation>,
 }
 
 impl Lexer /*<'a>*/ {
@@ -98,6 +112,7 @@ impl Lexer /*<'a>*/ {
             prev_token: Token::new(TokenKind::BOF, "", 0, 0),
             lineno_token_starts: 0,
             col_token_starts: 0,
+            interpol_stack: vec![Interpolation::Not],
         }
     }
 
@@ -112,6 +127,7 @@ impl Lexer /*<'a>*/ {
             prev_token: Token::new(TokenKind::BOF, "", 0, 0),
             lineno_token_starts: 0,
             col_token_starts: 0,
+            interpol_stack: vec![Interpolation::Not],
         }
     }
 
@@ -623,32 +639,94 @@ impl Lexer /*<'a>*/ {
         Ok(self.emit_token(kind, &cont))
     }
 
-    fn lex_str(&mut self) -> LexResult<Token> {
+    fn str_line_break_error(token: Token, line: usize) -> LexError {
+        LexError::syntax_error(
+            line,
+            token.loc(),
+            switch_lang!(
+                "japanese" => "文字列内で改行をすることはできません",
+                "simplified_chinese" => "在一个字符串中不允许有换行符",
+                "traditional_chinese" => "在一個字符串中不允許有換行符",
+                "english" => "Line breaks are not allowed within a string",
+            ),
+            Some(
+                switch_lang!(
+                    "japanese" => "\"\"内で改行を使いたい場合は'\\n'を利用してください",
+                    "simplified_chinese" => "如果你想在\"\"中使用换行符,请使用'\\n'",
+                    "traditional_chinese" => "如果你想在\"\"中使用換行符,請使用'\\n'",
+                    "english" => "If you want to use line breaks within \"\", use '\\n'",
+                )
+                .into(),
+            ),
+        )
+    }
+
+    fn invalid_escape_error(ch: char, token: Token) -> LexError {
+        LexError::syntax_error(
+            0,
+            token.loc(),
+            switch_lang!(
+                "japanese" => format!("不正なエスケープシーケンスです: \\{}", ch),
+                "simplified_chinese" => format!("不合法的转义序列: \\{}", ch),
+                "traditional_chinese" => format!("不合法的轉義序列: \\{}", ch),
+                "english" => format!("illegal escape sequence: \\{}", ch),
+            ),
+            None,
+        )
+    }
+
+    fn unclosed_string_error(token: Token, by: &str, line: usize) -> LexError {
+        let by = if by.is_empty() {
+            "".to_string()
+        } else {
+            switch_lang!(
+                "japanese" => format!("\"\"\"によって"),
+                "simplified_chinese" => format!("\"\"\"关"),
+                "traditional_chinese" => format!("\"\"\"关"),
+                "english" => format!("by \"\"\""),
+            )
+        };
+        LexError::syntax_error(
+            line,
+            token.loc(),
+            switch_lang!(
+                "japanese" => format!("文字列が{by}閉じられていません"),
+                "simplified_chinese" => format!("字符串没有被{by}闭"),
+                "traditional_chinese" => format!("字符串没有被{by}闭"),
+                "english" => format!("the string is not closed {by}"),
+            ),
+            None,
+        )
+    }
+
+    fn unclosed_interpol_error(token: Token) -> LexError {
+        LexError::syntax_error(
+            0,
+            token.loc(),
+            switch_lang!(
+                "japanese" => "文字列内の補間が閉じられていません",
+                "simplified_chinese" => "字符串内的插值没有被闭",
+                "traditional_chinese" => "字符串內的插值沒有被閉",
+                "english" => "the interpolation in the string is not closed",
+            ),
+            None,
+        )
+    }
+
+    fn lex_single_str(&mut self) -> LexResult<Token> {
         let mut s = "\"".to_string();
         while let Some(c) = self.peek_cur_ch() {
             match c {
-                '\n' => {
-                    let token = self.emit_token(Illegal, &s);
-                    return Err(LexError::syntax_error(
-                        0,
-                        token.loc(),
-                        switch_lang!(
-                            "japanese" => "文字列内で改行をすることはできません",
-                            "simplified_chinese" => "在一个字符串中不允许有换行符",
-                            "traditional_chinese" => "在一個字符串中不允許有換行符",
-                            "english" => "Line breaks are not allowed within a string",
-                        ),
-                        Some(
-                            switch_lang!(
-                                "japanese" => "\"\"内で改行を使いたい場合は'\\n'を利用してください",
-                                "simplified_chinese" => "如果你想在\"\"中使用换行符,请使用'\\n'",
-                                "traditional_chinese" => "如果你想在\"\"中使用換行符,請使用'\\n'",
-                                "english" => "If you want to use line breaks within \"\", use '\\n'",
-                            )
-                            .into(),
-                        ),
-                    ));
-                }
+                '\n' => match self.interpol_stack.last().unwrap() {
+                    Interpolation::SingleLine if self.interpol_stack.len() == 1 => {
+                        let token = self.emit_token(Illegal, &s);
+                        return Err(Self::str_line_break_error(token, line!() as usize));
+                    }
+                    _ => {
+                        let token = self.emit_token(Illegal, &s);
+                        return Err(Self::unclosed_interpol_error(token));
+                    }
+                },
                 '"' => {
                     s.push(self.consume().unwrap());
                     let token = self.emit_token(StrLit, &s);
@@ -659,6 +737,12 @@ impl Lexer /*<'a>*/ {
                     if c == '\\' {
                         let next_c = self.consume().unwrap();
                         match next_c {
+                            '{' => {
+                                s.push_str("\\{");
+                                self.interpol_stack.push(Interpolation::SingleLine);
+                                let token = self.emit_token(StrInterpLeft, &s);
+                                return Ok(token);
+                            }
                             '0' => s.push('\0'),
                             'r' => s.push('\r'),
                             'n' => s.push('\n'),
@@ -668,17 +752,7 @@ impl Lexer /*<'a>*/ {
                             '\\' => s.push('\\'),
                             _ => {
                                 let token = self.emit_token(Illegal, &format!("\\{next_c}"));
-                                return Err(LexError::syntax_error(
-                                    0,
-                                    token.loc(),
-                                    switch_lang!(
-                                        "japanese" => format!("不正なエスケープシーケンスです: \\{}", next_c),
-                                        "simplified_chinese" => format!("不合法的转义序列: \\{}", next_c),
-                                        "traditional_chinese" => format!("不合法的轉義序列: \\{}", next_c),
-                                        "english" => format!("illegal escape sequence: \\{}", next_c),
-                                    ),
-                                    None,
-                                ));
+                                return Err(Self::invalid_escape_error(next_c, token));
                             }
                         }
                     } else {
@@ -691,17 +765,7 @@ impl Lexer /*<'a>*/ {
             }
         }
         let token = self.emit_token(Illegal, &s);
-        Err(LexError::syntax_error(
-            0,
-            token.loc(),
-            switch_lang!(
-                "japanese" => "文字列が\"によって閉じられていません",
-                "simplified_chinese" => "字符串没有被\"关闭",
-                "traditional_chinese" => "字符串没有被\"关闭",
-                "english" => "the string is not closed by \"",
-            ),
-            None,
-        ))
+        Err(Self::unclosed_string_error(token, "\"", line!() as usize))
     }
 
     fn lex_multi_line_str(&mut self) -> LexResult<Token> {
@@ -712,11 +776,21 @@ impl Lexer /*<'a>*/ {
                 let next_c = self.peek_cur_ch();
                 let aft_next_c = self.peek_next_ch();
                 if next_c.is_none() {
-                    return self._unclosed_multi_string(&s);
+                    let token = self.emit_token(Illegal, &s);
+                    return Err(Self::unclosed_string_error(
+                        token,
+                        "\"\"\"",
+                        line!() as usize,
+                    ));
                 }
                 if aft_next_c.is_none() {
                     s.push(self.consume().unwrap());
-                    return self._unclosed_multi_string(&s);
+                    let token = self.emit_token(Illegal, &s);
+                    return Err(Self::unclosed_string_error(
+                        token,
+                        "\"\"\"",
+                        line!() as usize,
+                    ));
                 }
                 if next_c.unwrap() == '"' && aft_next_c.unwrap() == '"' {
                     self.consume().unwrap();
@@ -725,6 +799,7 @@ impl Lexer /*<'a>*/ {
                     let token = self.emit_token(StrLit, &s);
                     return Ok(token);
                 }
+                // else unclosed_string_error
                 s.push(c);
             } else {
                 let c = self.consume().unwrap();
@@ -732,6 +807,12 @@ impl Lexer /*<'a>*/ {
                     '\\' => {
                         let next_c = self.consume().unwrap();
                         match next_c {
+                            '{' => {
+                                s.push_str("\\{");
+                                self.interpol_stack.push(Interpolation::MultiLine);
+                                let token = self.emit_token(StrInterpLeft, &s);
+                                return Ok(token);
+                            }
                             '0' => s.push('\0'),
                             'r' => s.push('\r'),
                             '\'' => s.push('\''),
@@ -746,24 +827,14 @@ impl Lexer /*<'a>*/ {
                             }
                             _ => {
                                 let token = self.emit_token(Illegal, &format!("\\{next_c}"));
-                                return Err(LexError::syntax_error(
-                                    0,
-                                    token.loc(),
-                                    switch_lang!(
-                                        "japanese" => format!("不正なエスケープシーケンスです: \\{}", next_c),
-                                        "simplified_chinese" => format!("不合法的转义序列: \\{}", next_c),
-                                        "traditional_chinese" => format!("不合法的轉義序列: \\{}", next_c),
-                                        "english" => format!("illegal escape sequence: \\{}", next_c),
-                                    ),
-                                    None,
-                                ));
+                                return Err(Self::invalid_escape_error(next_c, token));
                             }
                         }
                     }
                     '\n' => {
                         self.lineno_token_starts += 1;
                         self.col_token_starts = 0;
-                        s.push('\n')
+                        s.push('\n');
                     }
                     _ => {
                         s.push(c);
@@ -774,25 +845,120 @@ impl Lexer /*<'a>*/ {
                 }
             }
         }
-        self._unclosed_multi_string(&s)
+        let token = self.emit_token(Illegal, &s);
+        if self.interpol_stack.len() == 1 {
+            Err(Self::unclosed_string_error(
+                token,
+                "\"\"\"",
+                line!() as usize,
+            ))
+        } else {
+            Err(Self::unclosed_interpol_error(token))
+        }
     }
 
-    // for multi-line strings unclosed error
-    fn _unclosed_multi_string(&mut self, s: &str) -> LexResult<Token> {
-        let col_end = s.rfind('\n').unwrap_or_default();
-        let error_s = &s[col_end..s.len() - 1];
-        let token = self.emit_token(Illegal, error_s);
-        Err(LexError::syntax_error(
-            0,
-            token.loc(),
-            switch_lang!(
-                "japanese" => "文字列が\"\"\"によって閉じられていません",
-                "simplified_chinese" => "字符串没有被\"\"\"关闭",
-                "traditional_chinese" => "字符串没有被\"\"\"关闭",
-                "english" => "the string is not closed by \"\"\"",
-            ),
-            None,
-        ))
+    /// e.g. `}aaa"`, `}aaa{`
+    fn lex_interpolation_mid(&mut self) -> LexResult<Token> {
+        let mut s = "}".to_string();
+        while let Some(c) = self.peek_cur_ch() {
+            match c {
+                '\n' => match self.interpol_stack.last().unwrap() {
+                    Interpolation::MultiLine => {
+                        self.lineno_token_starts += 1;
+                        self.col_token_starts = 0;
+                        self.consume().unwrap();
+                        s.push('\n');
+                    }
+                    Interpolation::SingleLine => {
+                        if self.peek_next_ch().is_some() {
+                            let token = self.emit_token(Illegal, &s);
+                            return Err(Self::str_line_break_error(token, line!() as usize));
+                        } else {
+                            let token = self.emit_token(Illegal, &s);
+                            return Err(Self::unclosed_string_error(token, "", line!() as usize));
+                        }
+                    }
+                    Interpolation::Not => {
+                        let token = self.emit_token(Illegal, &s);
+                        return Err(Self::unclosed_interpol_error(token));
+                    }
+                },
+                '"' => {
+                    s.push(self.consume().unwrap());
+                    match self.interpol_stack.last().unwrap() {
+                        Interpolation::MultiLine => {
+                            let next_c = self.peek_cur_ch();
+                            let aft_next_c = self.peek_next_ch();
+                            if next_c.is_none() {
+                                self.interpol_stack.pop();
+                                let token = self.emit_token(Illegal, &s);
+                                return Err(Self::unclosed_string_error(
+                                    token,
+                                    "\"\"\"",
+                                    line!() as usize,
+                                ));
+                            }
+                            if aft_next_c.is_none() {
+                                self.interpol_stack.pop();
+                                s.push(self.consume().unwrap());
+                                let token = self.emit_token(Illegal, &s);
+                                return Err(Self::unclosed_string_error(
+                                    token,
+                                    "\"\"\"",
+                                    line!() as usize,
+                                ));
+                            }
+                            if next_c.unwrap() == '"' && aft_next_c.unwrap() == '"' {
+                                self.interpol_stack.pop();
+                                self.consume().unwrap();
+                                self.consume().unwrap();
+                                s.push_str("\"\"\"");
+                                let token = self.emit_token(StrInterpRight, &s);
+                                return Ok(token);
+                            }
+                            // else unclosed_string_error
+                        }
+                        Interpolation::SingleLine => {
+                            self.interpol_stack.pop();
+                            let token = self.emit_token(StrInterpRight, &s);
+                            return Ok(token);
+                        }
+                        Interpolation::Not => {}
+                    }
+                }
+                _ => {
+                    let c = self.consume().unwrap();
+                    if c == '\\' {
+                        let next_c = self.consume().unwrap();
+                        match next_c {
+                            '{' => {
+                                s.push_str("\\{");
+                                let token = self.emit_token(StrInterpMid, &s);
+                                return Ok(token);
+                            }
+                            '0' => s.push('\0'),
+                            'r' => s.push('\r'),
+                            'n' => s.push('\n'),
+                            '\'' => s.push('\''),
+                            '"' => s.push('"'),
+                            't' => s.push_str("    "), // tab is invalid, so changed into 4 whitespace
+                            '\\' => s.push('\\'),
+                            _ => {
+                                let token = self.emit_token(Illegal, &format!("\\{next_c}"));
+                                return Err(Self::invalid_escape_error(next_c, token));
+                            }
+                        }
+                    } else {
+                        s.push(c);
+                        if Self::is_bidi(c) {
+                            return Err(self._invalid_unicode_character(&s));
+                        }
+                    }
+                }
+            }
+        }
+        let token = self.emit_token(Illegal, &s);
+        Err(Self::unclosed_string_error(token, "", line!() as usize))
     }
 
     fn lex_raw_ident(&mut self) -> LexResult<Token> {
@@ -878,7 +1044,13 @@ impl Iterator for Lexer /*<'a>*/ {
             Some('[') => self.accept(LSqBr, "["),
             Some(']') => self.accept(RSqBr, "]"),
             Some('{') => self.accept(LBrace, "{"),
-            Some('}') => self.accept(RBrace, "}"),
+            Some('}') => {
+                if self.interpol_stack.last().unwrap().is_in() {
+                    Some(self.lex_interpolation_mid())
+                } else {
+                    self.accept(RBrace, "}")
+                }
+            }
             Some('<') => match self.peek_cur_ch() {
                 Some('.') => {
                     self.consume();
@@ -1145,7 +1317,7 @@ impl Iterator for Lexer /*<'a>*/ {
                             let token = self.emit_token(StrLit, "\"\"");
                             Some(Ok(token))
                         } else {
-                            Some(self.lex_str())
+                            Some(self.lex_single_str())
                         }
                     }
                     (Some(c), Some(next_c)) => {
@@ -1154,7 +1326,7 @@ impl Iterator for Lexer /*<'a>*/ {
                             self.consume(); // consume third '"'
                             Some(self.lex_multi_line_str())
                         } else {
-                            Some(self.lex_str())
+                            Some(self.lex_single_str())
                         }
                     }
                 }
diff --git a/compiler/erg_parser/parse.rs b/compiler/erg_parser/parse.rs
index 2d68a66f..1bc8c518 100644
--- a/compiler/erg_parser/parse.rs
+++ b/compiler/erg_parser/parse.rs
@@ -548,6 +548,7 @@ impl Parser {
         match self.peek() {
             Some(t)
                 if t.category_is(TC::Literal)
+                    || t.is(StrInterpLeft)
                     || t.is(Symbol)
                     || t.category_is(TC::UnaryOp)
                     || t.is(LParen)
@@ -1442,6 +1443,13 @@ impl Parser {
                 self.level -= 1;
                 Ok(Expr::Lit(lit))
             }
+            Some(t) if t.is(StrInterpLeft) => {
+                let str_interp = self
+                    .try_reduce_string_interpolation()
+                    .map_err(|_| self.stack_dec())?;
+                self.level -= 1;
+                Ok(str_interp)
+            }
             Some(t) if t.is(AtSign) => {
                 let decos = self.opt_reduce_decorators()?;
                 let expr = self.try_reduce_chunk(false, in_brace)?;
@@ -2161,6 +2169,87 @@ impl Parser {
         }
     }
 
+    /// "...\{, expr, }..." ==> "..." + str(expr) + "..."
+    /// "...\{, expr, }..." ==> "..." + str(expr) + "..."
+    fn try_reduce_string_interpolation(&mut self) -> ParseResult<Expr> {
+        debug_call_info!(self);
+        let mut left = self.lpop();
+        left.content = Str::from(left.content.trim_end_matches("\\{").to_string() + "\"");
+        left.kind = StrLit;
+        let mut expr = Expr::Lit(Literal::from(left));
+        loop {
+            match self.peek() {
+                Some(l) if l.is(StrInterpRight) => {
+                    let mut right = self.lpop();
+                    right.content =
+                        Str::from(format!("\"{}", right.content.trim_start_matches('}')));
+                    right.kind = StrLit;
+                    let right = Expr::Lit(Literal::from(right));
+                    let op = Token::new(
+                        Plus,
+                        "+",
+                        right.ln_begin().unwrap(),
+                        right.col_begin().unwrap(),
+                    );
+                    expr = Expr::BinOp(BinOp::new(op, expr, right));
+                    self.level -= 1;
+                    return Ok(expr);
+                }
+                Some(_) => {
+                    let mid_expr = self.try_reduce_expr(true, false, false, false)?;
+                    let str_func = Expr::local(
+                        "str",
+                        mid_expr.ln_begin().unwrap(),
+                        mid_expr.col_begin().unwrap(),
+                    );
+                    let call = Call::new(
+                        str_func,
+                        None,
+                        Args::new(vec![PosArg::new(mid_expr)], vec![], None),
+                    );
+                    let op = Token::new(
+                        Plus,
+                        "+",
+                        call.ln_begin().unwrap(),
+                        call.col_begin().unwrap(),
+                    );
+                    let bin = BinOp::new(op, expr, Expr::Call(call));
+                    expr = Expr::BinOp(bin);
+                    if self.cur_is(StrInterpMid) {
+                        let mut mid = self.lpop();
+                        mid.content = Str::from(format!(
+                            "\"{}\"",
+                            mid.content.trim_start_matches('}').trim_end_matches("\\{")
+                        ));
+                        mid.kind = StrLit;
+                        let mid = Expr::Lit(Literal::from(mid));
+                        let op = Token::new(
+                            Plus,
+                            "+",
+                            mid.ln_begin().unwrap(),
+                            mid.col_begin().unwrap(),
+                        );
+                        expr = Expr::BinOp(BinOp::new(op, expr, mid));
+                    }
+                }
+                None => {
+                    self.level -= 1;
+                    let err = ParseError::syntax_error(
+                        line!() as usize,
+                        expr.loc(),
+                        switch_lang!(
+                            "japanese" => "文字列補間の終わりが見つかりませんでした",
+                            "english" => "end of string interpolation not found",
+                        ),
+                        None,
+                    );
+                    self.errs.push(err);
+                    return Err(());
+                }
+            }
+        }
+    }
+
     /// x |> f() => f(x)
     fn try_reduce_stream_operator(&mut self, stack: &mut Vec<ExprOrOp>) -> ParseResult<()> {
         debug_call_info!(self);
diff --git a/examples/helloworld.er b/examples/helloworld.er
index 6dc7837f..e989381e 100644
--- a/examples/helloworld.er
+++ b/examples/helloworld.er
@@ -3,5 +3,6 @@ print! "こんにちは、世界！"
 print! "Γειά σου Κόσμε!"
 print! "!مرحبا بالعالم"
 
-greeting = "Hello"
-print! "{greeting}, world!"
+greetings = ["Good morning", "Hello", "Good evening"]
+for! greetings, greeting =>
+    print! "\{greeting}, world!"