Using Rutherford backscattering (RBS)/channeling method, we investigated the microstructure evolution in oxydizing atmosphere of Ni/Au contact on p-GaN during annealing at constant temperature (500 ℃) but for different alloying time. The technique of RBS/channeling and simulation of RUMP program were used to investigate the diffusion of the electrode metal. It was found that Ni diffused to the interior of the sample, Au diffused to the surface of p-GaN after 60s alloying time and the epitaxial structures of Au also began to form on p-GaN at the same time. The O also diffused into the sample after 180 s alloying time. Considering a continuous decrease in the ρc, it is suggested that the epitaxial structures of Au on p-GaN and the formation of NiO play a critical role in forming low resistance ohmic contact to p-GaN. At 300 s alloying time at the same annealing temperature (500℃), the epitaxial structure of Au was impoved further and the O also diffused deeper into the sample, and the ohmic contact reached the optimal value.