Creep-fatigue tests were conducted with 2 1/4 Cr-1Mo steel at 550°C in air. The results were analyzed with a damage model developed by the authors for an evaluation of the pure creep-fatigue behavior of the steel. The model is composed of the overstress concept and consists of two kinds of damages, that is, the time-independent damage and the time-dependent damage. The environmental effect is included in both damages. The environmental effect increases the time-independent damage, however, it decreases the time-dependent damage.