Theoretically, motor efficiency is impacted by motor load as well as frequency. The motor equivalent circuit method provides an effective method to estimate motor efficiency. However, the circuit parameters normally have to be determined through field tests, which are impossible to conduct when systems are in service. Based on the fact that measured motor efficiency and power factor data at four load levels under the rated frequency are always published, an iterative method is proposed to identify the circuit parameters using published data. The circuit parameters are used to estimate the motor efficiency under different frequencies. In this paper, the mathematical relations between motor efficiency and power-factor data and the circuit parameters are first formularized. then the least-squares method is utilized to identify the circuit parameters using the published data under the rated frequency, and finally the motor efficiency is estimated under different frequencies.