in order to understand the environmental effects on cyclic fatigue, static and cyclic fatigue behaviour was investigated in air and in vacuum for normally sintered silicon nitride and alumina. The cyclic fatigue lifetime in vacuum is considerably longer than that in air, indicating a remarkable stress corrosion cracking effect in the latter, especially in alumina. In addition, the cyclic loading effect in vacuum is almost the same in silicon nitride relatively insensitive to environmental effects and alumina susceptible environmental effects. From such results, it has been found that cyclic fatigue in air is approximately expressed as the superposition of pure cyclic loading effect, which is defined as cyclic loading effect in vacuum, and environmental effect. This relation was applied to some kinds of ceramics with different values of fracture toughness or different microstructures and the results obtained were discussed.