[en] Apart from superior creep strength and good oxidation resistance, thermal fatigue is one of the major concerns with superalloys as it is well established that a great number of failures occur because of thermal fatigue. The thermal fatigue life has been conventionally defined as the number of thermal cycles needed to form a major crack depth of 1 mm. The phenomenon of thermal fatigue has been extensively studied by earlier investigators and is found to be affected by the mechanical properties and microstructural characteristics as well as the test parameters. There has, however, been very little understanding of the microstructural changes brought about by thermal fatigue. The purpose of the present investigation, therefore, is to evaluate the changes in microstructure due to thermal fatigue in a Nimonic 263 alloy