The three part fractures of proximal humerus are not uncommon and the management is still under debate. For the good functional result, rigid internal or external fixation and early ROM exercise is mandatory. From Oct, 1989 to Oct, 1991, the results of treatment of 13 cases of proximal humerus 3-part fractures exclusively treated by internal fixation following the principles of the AO/ASIF group were analyzed at the Department of Orthopedic Surgery, Asan Medical Center. The results were as follows ; 1. The major cause of injury was traffic accident(9 out of 13 cases). 2. The associated injuries were brachlal plexopathy(1), rib fractures(1) and femur fractures (1). 3. The types of fractures were type B(7 cases), and type C(6 cases) according to AO/ASIF classification. 4. Open reduction and internal fixation was performed with T plate and tension band wiring, if necessary. 5. The excellent results were 4 cases and satisfactory results were 5 cases according to Neer criterla.