X20CrMoV12 1 steels were exposed at three different high temperatures and various durations. Aging parameter, LMP s, was used to describe long-term thermal history. Creep rupture, hardness and tensile tests for aged X20CrMoV12 1 steels were carried out. Yield strength was slightly decreased and hardness was rapidly decreased afterLMP s reached 20.6. Creep strength of aged X20CrMoV12 1 steel decreased rapidly after the LMPs reached 20.6. It was shown that the creep strength decreased due to the coarsening of acicular type M23C6 at subgrain boundaries and the agglomeration of M23C6 at prior austenite grain boundaries. After LMPs reached 20.6, the assumed linear regression line of stress and Larson-Miller parameter,LMPf, also moved toward a lower LMPf region as aging parameter increased. The dependency of creep rupture life on the microstructural degradation could be represented by CLMPs, which is LMPf at stress of 1 kg/mm2. It was found that the creep rupture life of X20CrMoV12 1 steel could be predicted by considering the material degradation during and prior to creep test.