As an alternative to an electromagnetic motor, a rotary traveling wave type ultrasonic motor (USM) has been attracted considerable attention. The rotary traveling wave type is more commonly used in industrial applications. In this paper, the USM impressed high load torque in low speed range is modeled using an equivalent circuit and a new torque control method is proposed to implement for a robot manipulator. Firstly, the USM is modeled using the equivalent circuit based on a piezoelectric equation and a mechanical model. In mechanical approach, the ratio of vibration speed of the vibrator to rotational speed of the rotor is assumed as 1: n, where n is a smaller variable than 1. According to this proposed equivalent circuit, if vibration amplitude of the vibrator is constant, output torque is proportional to voltage magnitude applied to piezoelectric ceramics and rotational speed of the rotor. Secondly, three parameters of two sinusoidal voltages: frequency, magnitude and phase difference are determined to control output torque. Finally, an experiment is conducted to verify the equivalent circuit model. Experimental results show that output torque of the USM is proportional to applied voltage magnitude. Therefore, the proposed model is shown to be valid and will be applied to robot manipulators.