In this paper, being based on Fourier expansion and energy method, the analytical expression of cogging torque of permanent magnet (PM) motor is derived, which can be used to analyze the effects of design parameters on cogging torque. Then a new method for reducing cogging torque by adopting semi circled magnetic pole is proposed. Finite element analysis is used to calculate the cogging torque and flux per pole for different shapes of magnetic pole. It is proved that the cogging torque could be greatly reduced by adopting semi circled magnetic pole, while the flux per pole changes little, which verifies the effectiveness of the proposed method.