Nd--sialons with the stoichiometric composition and extra 2wt.% addition of Nd2O3 in Nd0.333Si10Al-2ON15 were synthesized by hot-press sintering. The longtime oxidation resistance of the two ceramics at high temperature was investigated. The oxidation process is very similar, i.e. the weight gain increases linearly at primary stage, and then tends to be constant with prolonging the oxidation time. The oxidation kinetic obeys the parabolic rate law. Inward diffusion of oxygen into ceramics and outward diffusion of metal cations is the main oxidation mechanism. Due to the variation of the content of the added rare earth oxide, the oxidation performance of the two Nd--sialon ceramics are also different.