Long term creep strength property of a Mod.9Cr-1Mo steel has been investigated in conjunction with changes in microstructure, in order to understand a degradation mechanism at the long term conditions. The predicted creep rupture lives from short term creep deformation data whose rupture lives are less than 1000 h, are consistent with the experimental data up to about 10000 h. In the long term region longer than about 10000 h, however, creep rupture lives become shorter than the predicted one with increase in time to rupture. In the specimens creep ruptured at short term region less than about 10000 h, homogeneous progress in recovery of tempered martensitic microstructure, such as increase in lath width and coarsening of subgrain, has been observed. On the other hand, remarkable progress in such recovery of the microstructure has been observed at the vicinity of prior austenite grain boundary for the specimen creep ruptured at long term region under the low stresses. Such preferential recovery along a prior austenite grain boundary can promote the beginning of tertiary creep under the low stress condition. It has been concluded that preferential recovery along a prior austenite grain boundary is a main factor of a degradation of the Mod.9Cr-1Mo steel during long term creep deformation.