A compensating circuit for compensating the terminal voltage on the armature in a sensorless type DC brushless motor apparatus is described. The driving circuit of the motor apparatus outputs a three-phase power, and each phase is connected with a compensating circuit. The compensating circuit has a reference voltage, a resistance device, a diode device, a differential amplifier and a processor. The two terminals of the resistance device are respectively connected to the reference voltage and the anode of the diode device. The cathode of the diode device is connected to an output of the three-phase power supply. The two input terminals of the differential amplifier are respectively connected to the two terminals of the resistance device. The processor accepts the signal output from the differential amplifier and then calculates and outputs a compensating voltage.