High temperature oxidation tests were carried out in a flowing O2-N2 mixture for gas-pressure-sintered Si3N4 containing Y2O3 and Al2O3 as sintering aids. The oxidation temperature and time were 1400°C and 15h, respectively and O2 concentration in the gas mixture ranged from 1 to 100vol%. The oxidation reaction was monitored every minute by measuring of the evolved N2 and NO gases with a quadrupole mass spectrometer. The oxidation rate expressed as the N2 evolution rate was in the order 1%O2>21%O2≥100%O2. In oxidation in high O2 concentration, oxidation kinetics obeyed the parabolic law and a good conformity was obtained between the amount of N2 evolved and the weight gain of the Si3N4 sample after the oxidation. At a low O2 concentration (1vol%), the oxidation rate was partly expressed by the first-order kinetics and a poor conformity was observed between the amount of N2 evolved and the weight gain. It should be noted that the conformity improved when the gas flow rate was greatly increased.