Abstract: The wear characteristics of a rotor frictional material of ultrasonic motor (USM) were investigated on a pin on disc wear tester and a simulating friction characteristic tester of USM under common sliding and USM testing conditions, respectively. The effects of contact pre pressure and load torgue on the wear properties of the rotor frictional material were examined. The wear mechanisms of the rotor frictional material were analyzed based on scanning electron microscopic and optical microscopic observations of the worn surface morphologies. It has been found that the wear characteristics of the rotor frictional material under USM testing condition are different from that under common sliding testing condition. Namely, transfer film is observed on the worn surface of the rotor frictional material subject to common sliding testing, while the worn surface under USM testing condition is smooth and has no transfer film thereon, which is characterized by plowing and fatigue peeling. These features varied with the pre pressure and load torgue, which was correlative to the absolute ratio S f , of the speed difference. It was supposed that S f could be used to express the degree of the relative sliding friction between the stator and rotor of the USM. At S f <0.3, the frictional material surface is relatively smoother and characterized by slight fatigue peeling wear. At S f >0.51~0.6, it is characterized by serious plowing.