Abstract: The initial oxidation behavior of nitride layer (U₂N_3+x) on uranium metal was investigated by in-situ X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) in oxygen atmosphere at room temperature. AES differential spectrum of uranium and U 4f, N 1s, O 1s spectra all show that UN_xO_y is formed during the oxidation of nitride layer. When exposuring to 18 L and 120 L oxygen it was observed by AES profile measurements that an oxide-nitrogen rich-nitride sandwich structure was formed on the surface of nitride layer. As the OPV mixing peak of nitrogen-rich film is much lower than that of the nitride and oxide layer, and N 1s peak shows the same trend as OPV peak, nitrides with higher N/U ratio may form in the nitrogen-rich layer. It is implied that during the oxidation of uranium sesiquinitride the N atom will be substituted by O atom and move to fill the vacancies of neighbor nitride crystal lattice, which increases the N/U ratio of neighbor field and prevents the O atom’s diffusion.