The effect of microstructures, such as grain size and internal pores, on the fatigue behavior of aluminum titanate (AT, Al2TiO5) ceramics was investigated at room temperature and 973 K. When the average grain size of the AT ceramics was decreased from 22 to 13 mu m, the flexural strength was approximately doubled at both temperatures. The decrease in the grain size is considered to reduce the size and amount of microcracks present at the grain boundaries. Fatigue deterioration of the ceramics was accelerated by applying cyclic stress. The cyclic fatigue behavior of the ceramics with smaller grains suggested control by a "grain bridging degradation mechanism" at both temperatures. However, that of the ceramics with larger grains was governed by the fatigue mechanism, depending on the testing temperatures. There was little effect from internal pores on the cyclic fatigue behavior of the AT ceramics at room temperature. On the other hand, at 973 K, the lifetime of the cyclic fatigue was decreased due to the presence of pores.