In impact dynamics, coefficient of restitution (COR) is an important parameter to calculate the impulse. Generally, COR has been assumed a constant value. However, COR varies according to the colliding condition. In order to apprehend this phenomenon, analysis of impact dynamics is conducted using a 3-DOF finger mechanism. The external impulse exerted on a wall or a cart by the mechanism is a function of the robot's geometry and dynamic parameters. In this paper, we derive an analytical model of COR with the 3DOF finger model colliding to a wall and a cart, and emphasize that the actual motion just after impact is not the same as what we expected unless the variable COR is considered. The effectiveness of proposed COR model is verified through wall and cart impact experiments using the finger mechanism.