The easy-going oxidation of silicon nitride (Si3N4) at high temperature greatly hampers its potential applications. Here, we explored the reaction mechanism between beta-Si3N4 and O-2 via density functional theory (DFT) calculation, which discloses that O atoms are preferentially adsorbed on the top of Si atoms and N-2 starts to be generated as the dominant gas product at 2/3 monolayer (ML) O coverage. The vacancies formed by N-2 removal attract the O adatoms to transfer to the site of the N vacancy, which accelerates the adsorption of O and the formation of Si-O bonds toward the growth of SiO2 product. The surface oxidation of beta-Si3N4 (0001) has been clarified by the unambiguous evolution of [SiN4-nOn] (n = 0-4) tetrahedrons going through from [SiN4] tetrahedron to [SiO4] tetrahedron, providing a deep insight into intrinsic oxidation process of Si3N4 ceramic.