X-ray photoelectron spectroscopy has been used to study the chemical structures of silicon nitride films prepared by plasma anodization. The Si//3N//4/Si interface was studied by chemical depth profiling. It is found that SiO//xN//y exists within 3 nm of the interface and at the surface, but with nearly ideal silicon nitride in the rest of the film. Nearly the same distribution of SiO//xN//y near the interface is found for all films with thickness range from 5. 6 to 15. 6 nm. Possible mechanism of plasma anodic nitridation which can explain these results are discussed.