Abstract: This paper describes a theoretical study, using dynamic principle, about the influence of the ultrasonic vibration on the frictional properties of ultrasonic driving. Ultrasonic vibration of stator surface points at the contact region of stator and rotor in ultrasonic motor(USM) is resolved into horizontal and vertical vibration and the effects of two direction vibration on USM driving are analyzed. It is proposed that the horizontal vibration produces friction driving force and vertical vibration affects horizontal driving effect. The role of vertical ultrasonic vibration of stator surface points in USM is equivalent to the role of ultrasonic vibration being perpendicular to sliding direction in common sliding test. The reasons are revealed that the vertical ultrasonic vibration results in decrease of sliding friction coefficient in ultrasonic driving. An experiment is performed to verify the anti-friction effect of vertical ultrasonic vibration.Experimental results show reasonable correlation betw een theory and experiments.