In the present work, the thermal performance of a compact two-phase flat thermosyphon is investigated numerically. A two-dimensional computational fluid dynamics model based on the volume of fluid method and Lee model is employed to investigate the heat transfer characteristics of a two-phase flat thermosyphon. The liquid-vapor interface is tracked using the volume of fluid approach, and the Lee model is used to model liquid-vapor phase change heat transfer. The numerical model is validated with the experimental results. The thermal performance of a flat copper thermosyphon with water as a working fluid is tested at a 53% filling ratio. The flat thermosyphon's total thermal resistance is 0.39 to 0.31 K/W for heat fluxes of 10 - 40 W/cm². The spreading resistance is around 0.13 K/W, and a uniform temperature on the condenser surface is obtained. The flat thermosyphon distributed heat flux uniformly by spreading the heat through conduction and liquidvapor phase change. It is observed that the flat thermosyphon can be used as an effective heat spreader to mitigate the hotspots and improve the heat sink performance.