Abstract: A mechanics model of ultrasonic motor using longitudinal and torsional mode is established, then the relation of pre pressure( F pre ),voltage for logitudinal vibration( V l ),voltage for torsional vibration( V t ),friction force( f ) and totque( T ) is discussed. A pre pressure is corresponding with maximum output torque under some conditions. The simple explanation is given too. At last, the computing results and experiment results are given and they are coincident.