The static fatigue and cyclic fatigue behaviour of Si3N4 ceramics in different environments (air, H2O and kerosene) were studied systematically. The results show that the stress corrosion index(n) under cyclic loading is almost the same in the various medias, and much higher than that in the same media under static loading. Mechanical cycling was deduced to introduce an additional failure mechanism that caused the cyclic fatigue life to be much shorter than static fatigue life.