The causes of substrate defer producing bit errors for magneto-optical disks are analysed, and the mathematical model for describing the relation of readout waveform diStribution and the foreign substance contained in substrates is establised. On the basis, an analysis of the relationships between runout signal distortion, bit error length and foreign substances is given by numerical method. It is shown that whether a foreign substance of some size causes bit errors or not depends on many factors, such as the distance of a foreign substance from the recording plane, and the distance of a foreign substance from the readout spot, as well as the readout threshold voltage.