This invention discloses a GaN semiconductor device comprising a substrate; a metal-rich nitride compound thin film on the substrate; a buffer layer formed on the metal-rich nitride compound thin film, and a semiconductor stack layer on the buffer layer wherein the metal-dominated nitride compound thin film covers a partial upper surface of the substrate. Because metal-rich nitride compound is amorphous, the epitaxial growth direction of the buffer layer grows upwards in the beginning and then turns laterally, and the epitaxy defects of the buffer layer also bend with the epitaxial growth direction of the buffer layer. Therefore, the probability of the epitaxial defects extending to the semiconductor stack layer is reduced and the reliability of the GaN semiconductor device is improved.