The traveling wave piezoelectric micromotor is driven by the resonant vibration of the stator through the friction force of contact surface. Therefore, the physical contacting behavior heavily determines the operation efficiency and stability of motors. Current studies on such a behavior mainly focusing on the motor structures with teeth on stator, cannot be utilized to investigate the driving mechanism of micro motors which have no teeth on any components. A new three-dimensional model in cylindrical coordinate system is proposed in this paper to study the actuation characteristics of the ultrasonic micromotor developed by MEMS technology. Both the radial variation and contact area change are considered in modeling the driving force of stator, and the relative velocity between contacting particles on stator and rotor are analyzed in detail to calculate the work done by the friction force. Finally, the micromotor working in B13 mode with a radius of only 2 mm is fabricated and tested. The experimental results show a great agreement with analytical analysis which exhibits a great significance to the performance optimization of micro motors.