A discontinuous-current-mode (DCM) boost power factor correction (PFC) converter features zero-current turn-on for the switch, no reverse recovery in diode, and constant-frequency operation. However, the input power factor (PF) is relatively low when the duty cycle is constant in a half line cycle. This paper derives the expressions of the input current and PF of the DCM boost PFC converter, and based on that, variable-duty-cycle control is proposed so as to improve the PF to nearly unity in the whole input-voltage range. A method of fitting the duty cycle is further proposed for simplifying the circuit implementation. Other than a higher PF, the proposed variable-duty-cycle control achieves a lower output-voltage ripple and a higher efficiency over constant-duty-cycle control. The experimental results from a 120-W universal input prototype are presented to verify the effectiveness of the proposed method.