There is summarized a series of creep-fatigue data on a normalized and tempered 2 1/4Cr-1Mo steel obtained at 550°C in a very high vacuum environment under a wide variety of a strain-time program. A special feature of a stress-strain response of the steel is described to give the overstress and the internal back stress. A damage equation is developed to predict a creep-fatigue behavior which is free from the environmental effect.