A torque output mathematics model of ultrasonic motor using longitudinal and torsional mode is presented. The relation of pre-pressure, excitation volt for longitudinal vibration, excitation volt for torsional vibration, friction force, torque output, speed of rotor, and vibration speed of torsional vibrator is discussed. The experiments show that the maximum torque output stops increasing and gets a maximum value when the excitation volt for torsional vibration reaches 300 V. The motor's maximum torque output is 1 N · m when Vt is 350 V. This phenomenon can be explained by the presented model. The simulation results and experiment results are coincident.