According to the characteristic of permanent magnet synchronous motor (PMSM), a novel stator flux observer based on Particle Filter (PF) is introduced in order to reduce the ripple at direct torque control (DTC) system. The standard Particle Filter algorithm is presented at first, and the application of the Particle Filter algorithm in the high performance Du: system of PMSM is primarily discussed. Terminal voltages and stator currents measured are used to estimate the stator flux on line according to the PF algorithm at real time, which can provide flux feedback for DTC system. At last, a simulated experiment is done. The simulation results indicate that the proposed algorithm can achieve high estimation precision, and the system has good properties.