Abstract: The self-correction USM can automatically eliminate accumulated error of rotor displacement or angular displacement for an encoderless motor to be used,its merits are small volume and simple construction.This paper first has studied the influence of the rotor distortion on self-correction function.The restore torque produced by distortion of rotor having slots in side was calculated by FEM. The new phenomena in self-correction USM was explained. And the measurments for two prototype USM were carrid out,the experement results are consistent with calculation results.