Abstract: A mechanical model of a longitudinally vibration ultrasonic motor is proposed. The working principle of the ultrasonic motor is expounded. The sticking and slipping between the stator and rotor in the intermittent contact region are analysed theoretically. The analytical expression of the motor's driving force that undergoes continuous changes is given. The characteristics of the ultrasonic motor, including the rotating speed of the rotor, the input energy, the output energy and the efficiency of the motor are discussed. The effect of the initial compressive force, driving frequency, load, and the moment of the inertia of the motor is examined. The numerical results obtained from this model coincide with those obtained from existing experiments. These results and conclusions provide reliable theoretical basis for the design, construct and control of ultrasonic motors.