This paper presents a method of intelligent perception assist (IPA) with optimum force vector modification for an upper-limb exoskeleton robot to assist the perception of the user to interact with environment in addition to power-assist. The exoskeleton assists not only the motion but also the perception of the user. In the case of perception assist, the exoskeleton assists user to interact with the environment by monitoring the user's interaction with the environment. The exoskeleton is equipped with a sonar sensor and a stereo camera to gain information on the user's interaction with the environment. When the user interacts with the environment, if the exoskeleton found any problem in the user's motion, the optimum force vector modification (i.e., the modification of power-assist force vector with the optimum amount of force) is carried out by the exoskeleton to modify the user's motion to solve the problem without giving any uncomfortable feeling to the user due to motion modification. IPA ensures that the user's interaction with the environment can be monitored by the camera all the time by rotating the camera to track the location of the user's hand. The effectiveness of the proposed concept was experimentally evaluated.