diff --git a/mrbgems/mruby-string-ext/test/string.rb b/mrbgems/mruby-string-ext/test/string.rb
index 0400a8da5..52ee2fcb6 100644
--- a/mrbgems/mruby-string-ext/test/string.rb
+++ b/mrbgems/mruby-string-ext/test/string.rb
@@ -726,3 +726,66 @@ assert('String#-@') do
   a = -(a.freeze)
   assert_true(a.frozen?)
 end
+
+assert('String#bytesplice') do
+  # range, replace (len1=len2)
+  a = "0123456789"
+  assert_equal "ab", a.bytesplice(1..2, "ab")
+  assert_equal "0ab3456789", a
+
+  # range, replace (len1>len2)
+  a = "0123456789"
+  assert_equal "ab", a.bytesplice(1..3, "ab")
+  assert_equal "0ab456789", a
+
+  # range, replace (len1<len2)
+  a = "0123456789"
+  assert_equal "ab", a.bytesplice(1..1, "ab")
+  assert_equal "0ab23456789", a
+
+  # idx, len, replace (len1=len2)
+  a = "0123456789"
+  assert_equal "ab", a.bytesplice(1, 2, "ab")
+  assert_equal "0ab3456789", a
+
+  # idx, len, replace (len1>len2)
+  a = "0123456789"
+  assert_equal "ab", a.bytesplice(1, 3, "ab")
+  assert_equal "0ab456789", a
+
+  # idx, len, replace (len1<len2)
+  a = "0123456789"
+  assert_equal "ab", a.bytesplice(1, 1, "ab")
+  assert_equal "0ab23456789", a
+
+  b = "abcdefg"
+  # range, replace, range (len1=len2)
+  a = "0123456789"
+  assert_equal b, a.bytesplice(1..2, b, 0..1)
+  assert_equal "0ab3456789", a
+
+  # range, replace, range (len1>len2)
+  a = "0123456789"
+  assert_equal b, a.bytesplice(1..3, b, 1..2)
+  assert_equal "0bc456789", a
+
+  # range, replace, range (len1<len2)
+  a = "0123456789"
+  assert_equal b, a.bytesplice(1..1, b, 2..3)
+  assert_equal "0cd23456789", a
+
+  # idx, len, replace, idx, len (len1=len2)
+  a = "0123456789"
+  assert_equal b, a.bytesplice(1, 2, b, 0, 2)
+  assert_equal "0ab3456789", a
+
+  # idx, len, replace, idx, len (len1>len2)
+  a = "0123456789"
+  assert_equal b, a.bytesplice(1, 3, b, 1, 2)
+  assert_equal "0bc456789", a
+
+  # idx, len, replace, idx, len (len1<len2)
+  a = "0123456789"
+  assert_equal b, a.bytesplice(1, 1, b, 2, 2)
+  assert_equal "0cd23456789", a
+end