Abstract: Rotating-beam fatigue tests of 07Cr12NiMoVNb steel were carried out at room temperature and 550℃. The results indicated that the damage behavior of the steel samples with different grain size is similar. The relation between room temperature fatigue strength and ordinary tensile strength is in accordance to ojuger's experiential formula. High-temperature fatigue strength is related to ordinary tensile strength, but the linear coefficient is different from that of room temperature fatigue. Grain size has less influence on fatigue strength at room temperature. Yet at 550℃, the steel with fine grain exhibits higher fatigue strength which is contrary to creep behavior. For creep behavior, the steel with roase grain exhibits higher strength.