A two-stage electronic ballast for metal halide (MH) lamps is presented in this paper. The front-stage is a two-output power factor correction AC-DC flyback converter. The post-stage is a half-bridge inverter, which is used to control lamp power and drives MH lamp with a low-frequency square waveform current to avoid acoustic resonance. In addition, the lamp power of steady state is maintained at a constant level by utilizing the current-sensing level-shifting technique. The proposed topology has the advantages, such as unity power factor, low THD, low cost, and constant output-power. Finally, a laboratory prototype of 35W MH lamp electronic ballast is implemented to verify the feasibility.