Abstract: A sensorless speed control method of a high speed motor utilizing a reverse electromotive force comprises the steps of: adjusting to 50% a positive duty of rectangular waves generated from a reverse electromotive force signal of the high speed motor after accelerating the high speed motor in an accelerated speed mode to a mode switch RPM (first duty adjusting step); measuring a pulse difference between a rectangular wave edge and a synchronous signal when a voltage is adjusted to allow an ideal current waveform to be formed, thereby setting up a sensorless mode switch condition; utilizing the pulse difference measured in the switch condition set-up step to switch the high speed motor from an accelerated mode to a sensorless mode; and switching the high speed motor at the mode switch step to sensorless mode and increasing the RPM of the high speed motor to a high speed to thereby adjust a positive duty of the rectangular wave.