Abstract: GaN thin films were successfully grown on the Ga-diffused Si （111） substrates throughnitriding Ga₂O₃ thin films deposited by r.f. magnetron sputtering, and the relationship between nitri-dation time and the crystal quality of the films was investigated. The results reveal that the as-grownfilms are polycrystalline hexagonal GaN with wurtzite structure by the two-step growth method, Ga-diffusion layer effectively avoids nitridation of the Si substrates and relaxes the thermal mismatchbetween GaN and Si substrates. Under the same condition of nitridation temperature, the diameter of GaN grains become larger and the crystal quality of GaN films is improved with the increasing nitrida-tion time.