Permanent magnet synchronous motor (PMSM) was considered as a high order nonlinear system due to the parameter uncertainties and external disturbances. Superior control performance could not be guaranteed by using internal model control (IMC). An exponential convergence controlbased observer was derived to estimate uncertainties. An internal model was set up under the condition that coupling term and back electromotive force were compensated completely. Then, a statespace model for observer was constructed based on output term of IMC and feedback current. Finally the prediction error was obtained to estimate uncertainties. The system was simulated by MATLAB / Simulink. Simulation results showed that by using improved internal model control (IIMC), the disturbance observer could realize the current compensation, suppress the current ripple, reduce the steadystate error, and improve the response of speed, reduce the fluctuation of speed error under load disturbance.