diff --git a/README.md b/README.md
index d4613c700..95733e2d5 100644
--- a/README.md
+++ b/README.md
@@ -38,6 +38,7 @@ There is no dependency on other mrbgems.
  - q : 64-bit signed, native endian (`int64_t`)
  - S : 16-bit unsigned, native endian (`uint16_t`)
  - s : 16-bit signed, native endian (`int16_t`)
+ - U : UTF-8 character
  - V : 32-bit unsigned, VAX (little-endian) byte order
  - v : 16-bit unsigned, VAX (little-endian) byte order
  - x : null byte