Loss of lower extremities has been one of the main problems in human life. Although most of the available knee devices are aesthetically acceptable, there is a necessity for lighter and more compact mechanisms, especially for younger amputees. This problem can be solved by the combining compliant mechanism design with traditional mechanism design methods. In this study, one group of the prosthetics that is known as the compliant knee mechanisms" is evaluated. At rst, the dierent knee mechanisms, such as fourand six-bar knee linkages are investigated to calculate the values of the control moments (actuator torque). Then, the suitable location (where the actuator torque is to be exerted) is determined to reduce the knee control moment. Finally, the compliant joints are employed to provide the improved designs. Furthermore, an optimization method is employed to determine the optimum values of stiness instead of using an experimental technique. The obtained results show that use of the compliant joints in the knee mechanisms reduces the values of the control moments, signicantly. In fact, the compliant members decrease the peak torques during the stance phase. Therefore, by applying a compliant joint, a higher energy eciency and lighter knee mechanism can be achieved for ambulation.