The oxidation mechanism of the not sintered pellets and sintered bodies of $Si_3N_4$ was investigated. in air over the temperature range of 800~130$0^{circ}C$. The $eta$-cristobalite was instantaneously formed and covered the particles of powder packed in the not sintered and weakly sintered porous $Si_3N_4$ bodies by molecular diffusion of oxygen through the porous Si3N4 bodies and an immediate oxidation. The diffusion of oxygen ion through the formed $eta$-cristobalite surface layer is assumed to control the further oxidation of the $Si_3N_4$ particles of the porous $Si_3N_4$ bodies. The diffusion coefficients and activation energies of oxygen ion through the $eta$-cristobalite layer were obtained by the use of a derived equation.