Microstructure evolution and the changes in mechanical properties of HR3 Csteel during long-term aging at650,700 and 750℃ were investigated.The precipitated phases of the aging steel included M23C6 carbides,Z-phase and a trace amount of Nb(C,N).The M23C6 carbides were distributed mainly at the grain boundary,while Z-phase was mainly inside the grains.Amounts of both M23C6 carbides and Z-phase during the aging process increased with increasing aging period and temperature.Coarsening of M23C6 carbides was influenced significantly by aging time and temperature,while the size of the Z-phase was relatively less affected by the aging time and temperature,which had a steady strengthening effect.Coarsening of the M23C6 carbides was the main reason for the decline in high temperature yield strength during long-term aging at 750℃.The M23C6 carbides were linked into a continuous chain along the grain boundary which accounted for the decrease of toughness during aging. Microstructure evolution and the changes in mechanical properties of HR3 Csteel during long-term aging at 650, 700 and 750 ° C were investigated. The precipitated phases of the aging steel included M23C6 carbides, Z-phase and a trace amount of Nb (C, N) The M23C6 carbides were distributed primarily at the grain boundary, while Z-phase was mainly inside the grains. Amounts of both M23C6 carbides and Z-phase during the aging process increased with increasing aging period and temperature. Coarsening of M23C6 carbides was influenced by aging time and temperature, while the size of the Z-phase was relatively less affected by the aging time and temperature, which had a steady strengthening effect. Coarseening of the M23C6 carbides was the main reason for the decline in high temperature yield strength during long-term aging at 750 ° C. The M23C6 carbides were linked into a continuous chain along the grain boundary which accounted for the decrease of toughness during aging.