The microstructure stability during high temperature long-term aging of material for power plants is critical for the material choice and safety of power plants. In this paper the microstructure stability and mechanical properties of alloy 617B after 720 ℃/10000 h, 800 ℃/2000 h and 850 ℃/100 h aging and short term aging at different temperature are studied. The results show that the microstructure stability of alloy 617B after aging at 720 ℃ for 10,000 is relatively poor, γ? phase coarsens and harmful phase precipitates on grain boundary. Aging temperature has important influence on the microstructure stability of alloy 617B as γ? phase coarsened rapidly with the increasing of aging temperature and the intragranular precipitates transferred. The hardness of alloy 617B is mainly influenced by the precipitation rule of γ? phase. There are zipper-like precipitates in grain during 720 ℃ aging while the structure of phase needs further study.