MR fluid is a fluid whose viscosity increases when the magnetic field is applied. When it applied to a damper, the damping force can be broadly controlled according to the applied magnetic field. In particular, conventional MR dampers applied to artificial joints implement damping forces according to two phases; stance phase : motion to support the feet on the ground, and swing phase : motion to step out. In case of stance phase, the damping force should be increased by applying a magnetic field to support the body. On the other hand, in case of swing phase, the damping force should be removed by not applying the magnetic field so that the prosthesis can be easily rotated by the motor. In this study, a special mechanism of MR damper is proposed which can derive On/Off mechanism that changes magnetic field by using permanent magnet only. The On/Off type permanent magnet MR damper applying mechanical mechanism is designed and analyzed to demonstrate the generation of optimal damping force which is required for the patient’s motion without control device.