Unfused olecranon epiphysis has been reported rarely. We found an unfused olecranon tip within joint and a few small spurs on the apposing olecranon fossa and removed it successfully through the arthroscope. We think that it is a part of secondary ossification center of the olecranon and tbat it was separated through the epiphyseal plate by repetitive impaction of the tip of the olecranon into the olecranon fossa during pitching. In conclusion, fusion of the intra-articular part(articular center) of the olecranon epiphysis can not occur by nature. Repetitive extension overload during pitching may cause symptoms, and could be treated with arthroscopic control.