This paper presents a constant power control circuit for a three-stage high intensity discharge (HID) electronic ballast. The three-stage electronic ballast is composed of a boost pre-regulator to achieve high power factor, a DC/DC buck converter to regulate lamp current with constant lamp power, and a full-bridge inverter to drive the HID lamp with low frequency AC square wave current. The buck converter operated in current mode utilizes current sense level-shift technique to achieve constant power output. The proposed constant power control circuit is easy to be designed and implemented for the three-stage HID electronic ballast. Finally, a laboratory prototype of a 70 W HID electronic ballast is implemented. The measured results show that the proposed ballast can be applied for various types of HID lamp with very low lamp power variation (less than 1.9 %).