Owing to the needs of rehabilitation, there exist many types of knee rehabilitation devices in the market. However, most of them are designed by the convenience of storage, easy operation, and other auxiliary functions. The real motion of human knees is, in general, less considered. Based on the experiment of human knee motion, this project applies the theory of synthesis and an optimization procedure towards motion centroid in order to search for the best mechanism which fits human knees. The resulted mechanism has been proved to be able to faithfully trace the paths generated by the experiment. Using this approach, it is hoped to reduce discomfort of patients in their recovery and improve the design of current rehabilitation devices.