In recent years, knee diseases are spread especially in elderly people. Since performing daily activities such as walking and running, the knee supports the weight of the body, there is more likely to be injured. This issue is more important for elderly people who have weak muscles and almost all elderly people suffer from knee pain. One way to help this people in order to move normally is to use a wearable device to aid the knee. In this article, a passive wearable robot will be designed to improve the strength of the elderly who suffers from the knee pain. The robot uses the compliance elements to increase the power of the knee joint in parts of a cycle. This robot will be developed based on a Stephenson II six-bar mechanism. Using this mechanism has the advantage of producing the similar motion to a knee. In other words, this mechanism produces the linear and rotational motions simultaneously. Additionally, more compliance elements can be added to improve the performance of the wearable robot. The optimal dimensions of the robot will be Through the kinematics analysis and also the derivation of the dynamics equations and the numerical validations of these equations, the performance of the robot will be considered. The performance of the robot mounted on the leg is compared with the human. Obtained results show that the less power is required when a wearable robot is used. This proves the merits of the designed robot to be used for the elderly.