High-temperature characteristics (especially high-temperature strength and oxidation resistance) of gas pressure sintered Si₃N₄ ceramics using Yb₂O₃-SiO₂ additives were evaluated. The high temperature characteristic of Si₃N₄ depended on the Yb₂O₃/SiO₂ molar ratio of additives. High 1500°C bending strength of 395 MPa was obtained for sintered bodies with 10.0 mass% addition of additives with Yb₂O₃/SiO₂ molar ratio of 1.0. Also, small weight change (13.2 g/m²) during oxidation at 1300°C for 300 h was obtained in this sintered body. This is considered to be due to the precipitation of Yb₄Si₂N₂O₇ and Yb₂Si₂O₇ phase in the grain boundary glass phase, as induced by a crystallization process.