A method for reducing the altitudinal errors and run-out of a spindle motor is disclosed. The method includes the following steps. First is to mount a material layer on the loading surface. Then, a surface treatment is applied to the material layer until the average run-out of the surface of the material layer is below 10.sup.-2 mm during the running of the spindle motor, and the distance between the surfaces of the material layer and one end of a shaft of the spindle motor achieves an expected value.