This paper presents a new method of back emf detection for sensorless brushless DC motor drive system. The theoretical analysis of the back emf sensor shows that the back emf signal of each floating phase can be extracted whether the pulse width modulation is on or off. The results show that this method can effectively suppress high frequency switching noise without filtering. In addition, it provides a very precise zero-crossing point. In this paper, the sensor scheme is implemented on the micro-controller of sensorless brushless DC motor. The micro-controller has been successfully applied to the aeromodelling machine. The application results verify the correctness of the theoretical analysis and prove that the micro-controller can provide good motor performance.