diff --git a/Library/OcStringLib/OcUnicodeLib.c b/Library/OcStringLib/OcUnicodeLib.c
index 3ecbae04..065f96ac 100755
--- a/Library/OcStringLib/OcUnicodeLib.c
+++ b/Library/OcStringLib/OcUnicodeLib.c
@@ -19,6 +19,35 @@
 #include <Library/OcStringLib.h>
 #include <Library/PcdLib.h>
 
+/**
+  Convert a Unicode character to upper case only if
+  it maps to a valid small-case ASCII character.
+
+  This internal function only deal with Unicode character
+  which maps to a valid small-case ASII character, i.e.
+  L'a' to L'z'. For other Unicode character, the input character
+  is returned directly.
+
+
+  @param  Char  The character to convert.
+
+  @retval LowerCharacter   If the Char is with range L'a' to L'z'.
+  @retval Unchanged        Otherwise.
+
+**/
+STATIC
+CHAR16
+InternalCharToUpper (
+  IN      CHAR16                    Char
+  )
+{
+  if (Char >= L'a' && Char <= L'z') {
+    return (CHAR16) (Char - (L'a' - L'A'));
+  }
+
+  return Char;
+}
+
 /**
   Performs a case insensitive comparison of two Null-terminated Unicode strings,
   and returns the difference between the first mismatched Unicode characters.
@@ -64,13 +93,13 @@ StriCmp (
   ASSERT (StrSize (FirstString) != 0);
   ASSERT (StrSize (SecondString) != 0);
 
-  UpperFirstString  = CharToUpper (*FirstString);
-  UpperSecondString = CharToUpper (*SecondString);
+  UpperFirstString  = InternalCharToUpper (*FirstString);
+  UpperSecondString = InternalCharToUpper (*SecondString);
   while ((*FirstString != '\0') && (*SecondString != '\0') && (UpperFirstString == UpperSecondString)) {
     FirstString++;
     SecondString++;
-    UpperFirstString  = CharToUpper (*FirstString);
-    UpperSecondString = CharToUpper (*SecondString);
+    UpperFirstString  = InternalCharToUpper (*FirstString);
+    UpperSecondString = InternalCharToUpper (*SecondString);
   }
 
   return UpperFirstString - UpperSecondString;
@@ -140,16 +169,16 @@ StrniCmp (
     ASSERT (Length <= PcdGet32 (PcdMaximumUnicodeStringLength));
   }
 
-  UpperFirstString  = CharToUpper (*FirstString);
-  UpperSecondString = CharToUpper (*SecondString);
+  UpperFirstString  = InternalCharToUpper (*FirstString);
+  UpperSecondString = InternalCharToUpper (*SecondString);
   while ((*FirstString != L'\0') &&
          (*SecondString != L'\0') &&
          (UpperFirstString == UpperSecondString) &&
          (Length > 1)) {
     FirstString++;
     SecondString++;
-    UpperFirstString  = CharToUpper (*FirstString);
-    UpperSecondString = CharToUpper (*SecondString);
+    UpperFirstString  = InternalCharToUpper (*FirstString);
+    UpperSecondString = InternalCharToUpper (*SecondString);
     Length--;
   }