This paper discusses the correlation between thermal fatigue and low-cycle fatigue at elevated temperature in terms of the change in microstructure of materials during the fatigue process. It substantiates the formula for life prediction in fatigue at elevated temperature which has been proposed by the author and co-workers with experimental proof. It is found that the characteristic feature of the low-cycle fatigue of a low-carbon steel at the test temperature above 500 C is taken as a diffusion controlled process characterized by the formation of subgrains in the change of microstructure, proved by observations with an optical microscope as well as X-ray diffraction techniques. The same feature is observed in the case of thermal fatigue where the upper bound of temperature in temperature cycling is close to or above 500 C. The temperature level of low-cycle fatigue giving the same fatigue life as a thermal fatigue testing, that is, the equivalent temperature of thermal fatigue is close to the upper bound of temperature of thermal fatigue. In contrast, for the test of thermal fatigue or low-cycle fatigue where such a high temperature is not included in either the range or the level of test temperature, the fatigue damage is taken as controlled by the slip mechanism. The equivalent test temperature is close to the mean of the upper and the lower bound of thermal fatigue.