Abstract: In order to deal with the negative effects of load disturbances and parametric perturbation on the field oriented control system of synchronous motors, an active disturbance rejection controller (ADRC) was transplanted into the speed control of synchronous motors. A novel flux observer based on ADRC was proposed to solve the problems such as the direct current bias and parametric perturbation of conventional flux observers. Simulation results show that the ADRC ensures very good robustness to load disturbances and parameter variation with fast response, small overshoot, and good static and dynamic performances. The improved flux observer not only restrains effectively the direct current bias and parametric perturbation, but enhances the precision of the flux.