Abstract: For eliminating chattering caused by traditional sliding mode observer used in sensorless control of permanent magnet synchronous motor, a sliding mode observer based on the second order sliding mode control method was proposed to estimate position and speed of rotor of permanent magnet synchronous motor. The position and speed of rotor were estimated separately by traditional sliding mode observer and the second order sliding mode observer in Matlab/Simulink software under ideal condition and parameter perturbation condition. The results show that the second order sliding mode observer can obtain smooth curves of estimated position and speed of rotor under the condition of no low pass filter with smaller estimation error, which has better observation precision and robustness.