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Since conventional above-knee prostheses are passive type devices with constant mechanical properties, the knee joint motions are not similar to that of normal persons. On the other hand, active type prostheses can improve the swing phase gait but these are expensive, heavy and consume large energy. Thus we propose a semi-active prosthesis using the rotary magnetorheological (MR) damper. The torque dissipation in the knee joint can be controlled by the magnetic field induced by the solenoid. A 3-DOF leg simulator has been developed to generate various hip motions and analyze the results of walking motions. The 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. We applied the repetitive controller in conjunction with the computed control law and PD control law. This algorithm reduces the RMS tracking error as the repetitions of the tracking. Moreover, the proposed prosthesis system is adaptable to walking speed.