Abstract: Ultrasonic motor is an unconventional motor driven by frictional force and high frequency vibration based on the principle of inverse piezoelectric effect. The high frequency vibration and minor amplitude between the stator and rotor have its own characteristics at friction interface, which is different from the traditional interface. The characteristics have significant effect on conversion efficiency, motor’s lifetime and the choice of frictional material. This paper presented the dynamic friction coefficient in normal, standing wave and the traveling wave measured by self-manufactured experiment structure. The influences of the relative speed, preload and temperature's impact on friction coefficient between the stator and rotor were analyzed. The relationship between friction coefficient and preload was explained from friction binomial and elastoplasticity contact state of the interface. The paper also explained the effect of friction reduction in the presence of ultrasonic vibrations both from standing wave and traveling wave. The impact of temperature on friction coefficient was also tested. Finally, this paper revealed a friction model for ultrasonic motor. The friction reduction for ultrasonic motor is a foundmental work, which is useful for design, modeling and controlling of ultrasonic motor.