1. An identification system for identifying a user by receiving a motion trajectory generated by the user, the identification system comprising: an image sensor for capturing a plurality of images of the motion trajectory generated by the user in a repetitive circular motion; a storage unit for storing motion vector information of a group of users including or not including the user generating the motion trajectory, wherein each of the motion vector information includes a trajectory of said repetitive circular motion; and a comparing unit for comparing the plurality of images with the motion vector information to identify the user according to a habitual pattern of the user of said repetitive circular motion, so that different users are differentiated according to different habitual patterns.