A motor driving circuit includes a rotation speed request generator, a motor driving signal generating unit, an inverter circuit, a position detecting circuit, a current detecting module, a rotation speed signal lookup module, an automatic leading angle controller, and a modulation signal generating circuit. When the rotation speed request signal indicates that a rotation speed of a motor is adjusted to a current rotation speed, the rotation speed signal lookup module queries a lookup table and generates a leading angle indication signal for indicating a current leading angle as a adjusting angle, and the automatic leading angle controller generates a phase adjusting signal, and the modulation signal generation circuit roughly adjusts a modulation waveform with the adjusting phase. The modulation signal generating circuit performs a fine adjustment on the modulation waveforms according to a phase difference, thereby making the current zero-crossing point near the BEMF zero point.