Since stator flux observation of permanent magnet synchronous motor(PMSM) is affected by resistance variation, a new stator flux observer of PMSM under the $\gamma-\delta$ rotating coordinate system is proposed. The virtual $\gamma-\delta$ rotating coordinate system is adopted to replace the traditional α-beta stationary coordinate system to do stator resistance dynamic identification by Lyapunov stability criterion. On the basis of this, a new stator flux observer is constructed. Experiment results show that the proposed stator flux observer is robust to resistance parameter perturbation and load disturbance, and the on-line accurate observation of stator flux is realized.