This paper presents the design of an assistive mechanism to be used for the rehabilitation of human arm. First the motions of two types of rehabilitation exercises are described. The motions are planar motions, so a planar four bar mechanism is designed. The synthesis problem is formulated as a three-position synthesis problem. Next, the actuation issue is addressed. Actuation via cables is preferred for better force transmission. The connection point of the cable to the mechanism is optimized considering the curvature of the coupler point curve. Finally, multiple pulleys are designed for enhancing force transmission.