diff --git a/scripts/string.cpc b/scripts/string.cpc
index 7997202..b3e4af4 100644
--- a/scripts/string.cpc
+++ b/scripts/string.cpc
@@ -194,3 +194,7 @@ FUNCTION ORD(s : CHAR) RETURNS INTEGER
     r <- PYTHON("_result=ord(s)", s)
     RETURN r
 ENDFUNCTION
+
+FUNCTION SUBSTRING(s : STRING, start : INTEGER, size : INTEGER) RETURNS STRING
+    RETURN MID(s, start, size)
+ENDFUNCTION
diff --git a/src/AST/insert_func.py b/src/AST/insert_func.py
index c76301d..d8db84e 100644
--- a/src/AST/insert_func.py
+++ b/src/AST/insert_func.py
@@ -231,6 +231,8 @@ def exe(self):
         c = parameters[0]
         if c[1] == 'CHAR':
             return (c[0].lower(), 'CHAR')
+        elif c[1] == 'STRING':
+            return (c[0].lower(), 'STRING')
         else:
             add_error_message(f'Function `{self.type}` expect `CHAR`, but found `{c[1]}`', self)
 
@@ -252,6 +254,8 @@ def exe(self):
         c = parameters[0]
         if c[1] == 'CHAR':
             return (c[0].upper(), 'CHAR')
+        elif c[1] == 'STRING':
+            return (c[0].upper(), 'STRING')
         else:
             add_error_message(f'Function `{self.type}` expect `CHAR`, but found `{c[1]}`', self)