Displaced fractures of olecranon in adults require operative treatment. The goal of treatment is to rigidly fix the fracture fragment, to anatomically restore the joint surface and to allow range of motioin early. Authors treated fractures of olecranon by open reduction and internal fixation using K-wire and tension band wiring technique in 38 adult patients from April 1989 to October 1995 at the Department of Orthopedic Surgery, Maryknoll Hospital. This article documents the study of relationship between the treatment result and involvement of articular surface, articular step-off and gap of articular surface. The results were as follows ; 1. The clinical results were good or fair in 31 cases(82%) according to the criteria by Helm et al. 2. Involvement of articular surface, step-off and gap of articular surface affected the results. 3. The clinical results were satisfactory in patients with postoperative step-off less than 2mm and postoperative gap of articular surface less than 2mm. 4. The clinical results were satisfactory in patients with articular involvement less than 60%.