Because conventional above-knee prostheses are passive type device with constant mechanical properties, knee joint motions are not similar to that of normal persons. On the other hand, active type prostheses can improve swing phase gait but these are expensive, heavy and consume large energy. So the semi-active type prostheses are presented. We propose to use the rotary MR damper. The torque dissipation in the knee joint can be controlled by the magnetic field induced by solenoid. The 3 DOF leg simulator has been also developed to generate various hip motions and analyze the results of walking motions. This simulator is useful in designing suitable prosthesis for the handicapped. Tracking control of knee joint angle was performed with this leg simulator. The experiment shows that the proposed prosthesis system has good performance in swing phase. As a controller, we applied the repetitive controller in conjunction with a computed control law and PD control law. This algorithm reduced RMS tracking error a s the repetitions of the tracking. Moreover, the proposed prosthesis system is adaptable to walking speed.