The surface composition of GaN grown by plasma-assisted MBE was investigated using X-ray photoelectron spectroscopy and Auger election emission spectroscopy(AES), while the depth profile was analyzed by AES with Ar iron sputtering. The experimental results indicated that unintentional doped semi-in salating films formed by plasma-assisted MBE had a rich-gallium surface. It was found that the impurity carbon was chiefly adsorbed onto the surface while oxygen diffused into the bulk to distribute in a certain depth. Consequently, the oxygen-related impurity band and an acceptor level could be simultaneously formed in the layer. This influenced the optical and electrical properties of the films.