Nowadays, the number of people with gait disorder as one of post-stroke symptom is gradually, and continuously increasing. For these people to have similar Activities of Daily Living (ADL) as people without gait disorder, active knee assistance is essential. This requirement is coming to the surface. The philosophy of assisting robots this paper presents is that assistant robots should only help people, while not disturbing the wearers’ natural movement. For this philosophy, robot hardware has to be well-constructed before application of any kind of control algorithm. Therefore, for flexion angle from 0 to 90 degrees, knee assistant robot hardware is proposed to secure natural movement of patellar and while minimizing force between tibia and femur in this paper.