The effect of TiO2 content on the oxidation of sintered bodies from the conventional Si3N4-Y2O3-Al2O3-AlN system was investigated. Sintered specimens composed of Si3N4, Y2O3, Al2O3, and AlN, with a ratio of 100 : 5 : 3 : 3 wt% and containing TiO2 in the range of 0 to 5 wt% to Si3N4, were fabricated at 1775 degrees C for 4 h at 0.5 MPa of N-2. Oxidation at 1200 to 1400 degrees C for a maximum of 100 h was performed in atmospheres of dry and wet air flows. The relation between weight gain and oxidation time was confirmed to obey the parabolic law. The activation energies decreased with TiO2 content. In the phases present in the specimens oxidized at 1300 degrees C for 100 h in dry air, Y3Al5O12 and TiN, which had existed before oxidation, disappeared. Alpha-cristobalite and Y2O3. 2TiO(2) (Y2T) appeared in their place and increased with increasing TiO2 content. In those oxidized at 1400 degrees C, alpha -cristobalite was dominant and very small amounts of Y2O3. 2SiO(2) and Y2T were contained. There was a tendency for more alpha -cristobalite to form in oxidation in wet air than in dry air. Therefore, moisture was confirmed to affect the crystallization of SiO2 formed during oxidation. Judging from the lower activation energy, the crystallization, and the pores formation, we concluded that the addition of TiO2 decreases oxidation resistance.