The microstructure of a Ni-based P/M superalloy FGH95 after long-term aging at 650 oC was observed by SEM and TEM and its microstructure stability was also investigated. The results show that the microstructure of the alloy is stable at 650 oC. The major precipitated phases of the alloy are γ′ phase and carbides of MC and M23C6 without TCP phase precipitating after aging for 500-5000 h. The γ′ phase is precipitated in the matrix in three kinds of sizes during different heat treatment processes. The size of bulk γ′ phase is about 1 to 3 μm, which is precipitated on grain boundary. The size of intragranular γ′ phase is about 500 nm, with the shape of butterfly, which changes a little in size, morphology and distribution in matrix during the aging process. And the smaller γ′ phase experiences a series of changes such as growing up, splitting and growing up again and splitting again in long-term aging. Its size is in a range of 60 to 200 nm. A kind of carbide M23C6 is found on the grain boundary in the alloy aged for 3000 h.