Abstract: The oxide layer grown on the A15 Nb3Ge surface after exposure to air is found to be mainly Nb2O5 from the observation of X-ray photoelectron spectroscopy, and no germanium oxide is detected. In order to investigate the initial oxidation mechanism of Nb3Ge, the adsorption of O atoms on Nb3Ge surface is performed within density functional theory (DFT). O atom prefers to bonding with Nb atom, and the bond is both ionic and covalent, which is consistent with the experiment. As the outer Nb oxidizes gradually, the inner Nb atoms begin to exchange with outer Ge atoms with the motivation of the contact potential, therefore niobium oxide congregates on the surface, also there is Ge accumulation under this oxide layer.