To investigate the effect of a small amount of impurity, especially Ca(CaO), on the oxidation of Si3N4 ceramics at high temperatures, two kinds of Si3N4 ceramics with different amounts of impurities were fabricated. The decrease in the strength of Si3N4 ceramics containing calcium as an impurity became severe when oxidation time was longer and temperature was higher. The decrease in the strength was caused by the formation of pits. It is estimated that pits are formed due to the high concentration of calcium in the oxidized layer near to the pits.