In this paper, the growth GaN epilayer on the Si substrate by a novel vacuum reaction method rather than metal-organic chemical vapor deposition and moleculau beam epitaxy is reported. The effects of growth temperature and annealing process on the photoluminescence (PL) of GaN epilayer were investigated. Annealing could weaken the PL and the GaN epilayer grown at 1050℃ exhibited the strongest PL. It was demonstrated in secondary ion mass spectroscopy that both gallium and nitrogen were distributed uniformly within the epilayer, while gallium was segregated on the surface of epilayer. The high carrier concentration (1.7×1018/cm3) was associated with the impurities of silicon and oxygen and native defects existing in the epilayer. In-situ cleaning was proved to be efficient for the removal of oxygen on the silicon substrate.