This paper presents a stator flux estimation method based on duty cycles of inverter switching. The stator flux is calculated by integrating the back electromotive force, which is the terminal voltage minus the voltage drop on the stator resistance. The terminal voltage mentioned above is calculated by the duty cycles of inverter switching and dc link voltage, while the duty cycles is derived in the digital control system of induction motor based on the DSP. Without employing voltage sensors to measure phase voltage, the proposed flux estimation method is cost effective and error reducing. The method is also effective in fault-tolerant operation when the motor works with two phases. Experimental results verify the effectiveness of the proposed method.