SiC powder was heated in air over the temperature range of 1100-135$0^{circ}C$. $eta$-cristobalite was formed to cover the surfaces of SiC particles by the reaction: $SiC(s)+20_2(g)=SiO_2(s)+CO_2(g)$. It is assumed that the diffusion of oxygen ion through the formed surface layer of $eta$-cristobalite controls the oxidation of the SiC particles. The diffusion coefficient of oxygen ion through the $eta$-cristobalite layer was obtained as the following equation: $D=3.84{ imes}10^{-17}$exp(-14.7/RT)