Modified 9Cr-1Mo steel is a heat-resistant steel developed for a steam generator in FBR (Fast Breeder Reactor) and it has been applied to various thermal power plants. Recently, it was found that the fatigue limit of this steel did not appear up to 108 cycles at temperatures higher than 500ºC. The reason for the disappearance of the fatigue limit was attributed to the change of the initially designed microstructure of the alloy. The initially dispersed fine lath martensitic texture disappeared at temperatures higher than 500oC, when the amplitude of the applied stress exceeded a certain critical value. In order to explicate the dominant factors of this change quantitatively, the change of the microstructure and the strength of the alloy were continuously observed by applying an intermittent fatigue and creep tests at elevated temperatures and EBSD analysis. It was found that there was a critical stress which caused the microstructure change at each test temperature higher than 500oC, and the activation energy of the change was determined as a function of temperature and the applied tensile stress. The dominant factor of the micro structure change was the stress-induced acceleration of the atomic diffusion of the component element of the alloy.