Abstract: FOH95 alloy has been put into use for more than ten years.The present work has studied the microstructure of FGH95 alloy after HIPing and heat treatment at different temperatures. The particle surface segregation and surface structure were investigated by using AES and ESCA. The changes of all-microstructure during HIPing were investigated by microscope, SEM, TEM and particularly by a detailed observationof STEM on extraction replica and metallic thin foils. It wss evidenced that when the powder was HIPed at above 1050℃ MC carbides precipitated discretely at ppb, and the space between two particles is much greater than the size of these carbides. After aging treatment a lot of MC carbides precipitated at ppb would be converted to film like M₂₃C₆ carbides, which is semi-continuously distributed at the ppb.The solvus temperature of γ'-phase was found to be affected by the HIPing pressure.The results of AES and ESCA analysis showed that there were segregation elements such as O, C, Cr, Ti and S on the powder particle surface, and that the particle surfaces were partialy oxidized.