For the purpose of decreasing the cogging torque of permanent magnet servo motors, electro-magnetic field analysis methods to consider the detailed magnetic characteristics of the magnetic core are proposed. The detailed magnetic characteristics mean the magnetic characteristics in the actual condition affected by the stress of shrink fitting or the magnetic anisotropy. In the speech, the methods to consider the detailed magnetic characteristics in the magnetic field analysis are described. And the validity of the described methods are evaluated by comparing the measured cogging torque with the analyzed cogging torque for the verification motor.