As one of the significant parameters to characterize the motor performance, motor efficiency changes under variable speed and load conditions. Although there are many methods to measure the partial load motor efficiency under the full speed condition, few of them are available to estimate the efficiency under partial speed conditions. In this paper, a new and original concept of "load ratio" is brought forward to describe the motor performances. A simple method is proposed to estimate the motor efficiency under variable speeds operations and partial load conditions using load ratio, speed ratio, rated motor power and rated motor efficiency. To validate the accuracy of this method, the partial loads full speed operation data from the MotorMaster+ database and the published data under the variable speed conditions from other researcher's tests are used for demonstration. The results indicate that the relative errors using the proposed method at full speed are within 5%. Under variable speed conditions, the relative error is within 10% when the speed ratio is above 0.5. (C) 2015 Elsevier Ltd. All rights reserved.