During the operation of ultrasonic motors, the series resonant frequency of the motor may drift due to factors such as load and temperature rise. In order to achieve precise control of the motor, it is necessary to accurately identify and track the series resonant frequency of the motor. A series resonant frequency identification method for ultrasonic motors based on Butterworth-Van Dyke (BVD) equivalent circuit model of the motor is proposed. The driving voltage and current of the motor are used as feedback signals, which are then processed to obtain a differential voltage. The phase difference between the driving voltage and the differential voltage is compared. And the relationship between the phase difference and the series resonant frequency is analyzed. A circuit implementation scheme is designed and a physical circuit is manufactured. The series resonant frequency of the prototype motor is then identified with the proposed method. Moreover, the series resonant frequency of the motor is also measured with impedance analyzer. The result shows that the frequency identified with the proposed method is in good agreement with the measured one, which verifies the accuracy of the proposed series resonant frequency identification method for ultrasonic motors.