Abstract: It is found that MgO/Si₃N₄ composite possesses a self-impedient performance to oxidation. A compact layer on the surface, which is an anti-oxidation and anti-corrosion layer, formed spontaneously in MgO/Si₃N₄ composite in the process of oxidation, and the thickness and density of the compact layer increase by adding Si or Al into the composite. On the basis of the results, the forming mechanism, the composition and structure of the compact layer of the composites with or without Si or Al were studied. The transform oxygen partial pressure was proposed as a standard for evaluating the escape intensity of gaseous SiO and Al₂O. It will show using the thermodynamics and the standard that gaseous SiO and Al₂O can produce inside the composite and they reoxidize and react with other oxides in the outside layer to form the compact layer.