The creep experiments of 2Cr11NiMoVNbNB steel were carried out at 550oC and 600oC. The creep damage laws of this material are discussed. An increase of the stress exponent with decreasing temperature was found experimentally and a modified secondary creep rate equation including in creep threshold stress was derived. Creep damage analyses are made under constant stress and under relaxed stress with creep threshold stress. The experimental results show that the damage value calculated with creep time is almost equal to that calculated with the creep rate, and an iteration method of calculating is proposed.