This study evaluates the oxidative resistance and the thermal stability in inert atmosphere of silicon nitride fibers. When the silicon nitride fibers were oxidized at 1000 °C for 1 h, they possessed the highest tensile strength (1.53 GPa) because of the surface defect healing effect of the glassy phase, whereas those oxidized at higher temperatures showed lower strength than the non-oxidized fibers. After treatment at 1200 °C in air, they retained 63% of strength, which indicates that the fiber might possess high serving life at the temperature lower than 1200 °C. Besides, the strength retention of the fibers after 1 h at 1450 °C in nitrogen remained 57%, showing good thermal stability. However, surface oxidation adversely affected the thermal stability of the fibers at high temperatures in nitrogen because of the predominant oxygen-containing phase. Above 1500 °C, crystallization of Si₃N₄ along with decomposition of SiN_xO_y occurred, resulting in a considerable decrease in weight and a catastrophic decrease in strength for the oxidized fibers.