We present a case report of a 15-year-old kendo (Japanese fencing) player who suffered a stress fracture of the ulnar styloid process. Exercise of the kendo requires the athlete to flex his non-dominant wrist repeatedly in an ulnar direction, and causes the disorder. Excision of the osteochondral fragment relieved the symptoms. This lesion is likely to occur with other sports or activities which demand similar motion of the wrists.