Oxidation in the presence of air and water vapor at high temperatures was studied for Si3N4 ceramics containing Y2O3 and Al2O3 as sintering aids. The test environments for this study consisted of air with 0, similar to 1.2, and 6.4 v/o H2O at temperatures from 1000 degrees C to 1350 degrees C. The oxidation exposure times were up to 500 hours. The presence of water vapor enhances oxidation and crystallization of the oxidation phases. Weight loss was observed for the oxidation in air or dry air because of Na contamination during the fabrication processing. The effect of applied stress on the growth of oxide scale is minimal, however, the applied stress resulted in deeper penetration of oxygen and pit formation in the oxide phase.