In order to assist the daily life motion of physically weak persons such as elderly persons, many kinds of power-assist robot have been developed. In the case of physically weak persons, perception ability to perceive the surrounding environment is often deteriorated also. A method of perception-assist has been proposed to assist not only the user's motion but also interaction of the user with his/her environment. Perception-assist automatically modifies the user's motion when the power-assist robot detects the possibility of accident such as collision between the user and some obstacles. It is useful function for physically weak persons' daily motion assist. However, since unintentional motion is automatically generated by the robot, regardless of the user's motion intension, the user might feel uncomfortableness. In this paper, another motion modification method is investigated to make natural motion modification in perception-assist. It is known that kinesthetic illusion is generated and subjects feel as if their antagonist muscle is elongated when vibration stimulation is added to tendon of antagonist muscle around a joint. The possibility of the motion modification using vibration stimulation is investigated in this study. Vibration stimulation is added to biceps brachii during the elbow joint motion. Some features of motion modification using vibration stimulation were obtained during the elbow joint motion by performing the experiments.