For applying ceramics to engine parts, a method to measure flexural strength of ceramics was studied. Alumina and mullite were evaluated in flexural strength at high temperature. The ring flexural test method was proposed to obtain flexural strength simply and the results were compared with the ones obtained from the three point bending measurement. It was discussed also to have the influence of ring size on the difference of the measured values. The flexural strength of alumina and mullite ceramics at high temperature was measured for two cases. In one case the specimens were cut out from the virgin materials and in the other from the corroded materials exposed to combustion gas of kerosene, heavy oil and kerosene with ash constituents.The results of these tests are summarized as follows:(1) The ring flexural test method can be used for the evaluation of flexural strength.(2) Alumina has the high flexural strength at the room temperature. But it decreases remarkably under the environment exposed to combustion gas at 600 °C to 1400 °C.(3) Mullite shows an unusual phenomenon that the flexural strength at 1000°C is more than twice as high as it at room temperature. Corrosion resistance is slightly good, but the strength level and heat resistance are low as compared with other ceramics and engineering materials.