A driver with high power factor, high efficiency and safety isolation are more attractive in the lighting application. Flyback is an ideal choice to be the power factor stage in LED driver. But one of the most difficulty in flyback topology is the energy stored in its leakage inductance. This part of the energy will introduce very high voltage spike on the main switch. A clamp circuit is needed.Passive clamp flyback converter is with few components and effectively clamping the voltage stress on transistor. But its switching frequency is limited by its switching loss. So the transformer volume in passive clamp flyback is very bulky. The aim of this project is to investigate high switching frequency Active Clamp Flyback converter with GaN transistor as power factor correction in lighting application.With the active clamp loop, the leakage inductance energy and snubber loss could be utilized properly to allow ZVS on switches under all line and load conditions and recycled to input source.So high switching frequency and high efficiency are both achievable. In this project,specific control method, dead time length and conduction mode are properly designed for active clamp flyback converter as PFC in lighting application.The implementation of closed control loop is not included.A 50W (75V) prototype of active clamp flyback front end converter operation around 1MHz with GaN transistor as Power Factor Correction stage in lighting application is developed to verify the analysis.