Walking is human's natural means to move from one place to another. Walking or gait cycle involves the motion of many of the body limbs and joints. Knee plays such an important role in the gait cycle. Motion of knee joint is not that simple kind of motion. It involves planer rolling and gliding motion as well as out of plane motion. Artificial limbs are used to enable amputees to function normally. Knee unit is a very critical component of the above knee prosthetic limb. . In this work an artificial knee was designed consisting of a four bar linkage in which motion was controlled by a suitable Electro-rheological (ER) damper. A complete analysis of the kinematics of motion for the four bars and the ER damper was made. Stability moment required to control the artificial knee during walking was obtained, and comparison between the performance of artificial and normal knees was elaborated.