The bipolar hemiarthroplasty is one of the treatment methods of the avascular necrosis of the femoral head and the fracture of the femur neck. The mechanism of bipolar hemiarthroplasty is intended to distribute the forces at the site of the inner and outer articulation, and theoritically diminish acetabular wear and produce a painless arthroplasty. The authors analyzed motion of two components in 23 cases of the bipolar hemiarthroplasty, performed at the Department of Orthopaedic Surgery, Korea University Hospital, between January, 1985 and January, 1993 with average 47 months follow up(range 24 to 84 months). The results obtained were as follows: 1. In the motion of the two components, inner bearing motion was average 11.5 degrees(from 1 to 29 degrees) and outer bearing motion was average 21.4 degrees(from 3 to 45 degrees). 2. The inner bearing motion was highest at 12 months with average 16 degrees and outer bearing motion was lowest during the same period with average 15 degrees. During the follow up the inner bearing motion was decreased whereas outer bearing motion was increased. 3. The inner or outer bearing motion of bipolar component was no correlation with age, sex and clinical result.