The effect of heat treatment on the microstructure and microhardness of a Ni–Fe based superalloy for700 °C advanced ultra-supercritical coalfi red power plants was investigated. Results showed that the main phases in the alloy were γ, γ′, MC and M_23C_6, and no harmful phase was observed in the alloy.M_23C_6-type carbides discretely distributed nearby grain boundaries as the alloy was aged at above840 °C. The microhardness decreased with increasing aging temperature. The coarsening of γ′ led to the increment of microhardness at 780 °C and 810 °C for a short aging time, and a signi fi cant decrease in microhardness after aging at 840 °C. The aging temperature had more signi fi cant role on the microstructure than holding time. Therefore, to obtain optimum strengthening effect for this alloy, the aging temperature should not exceed 810 °C. The effect of heat treatment on the microstructure and microhardness of a Ni-Fe based superalloy for 700 ° C advanced ultra-supercritical coalfi red power plants was investigated. Results showed that the main phases in the alloy were γ, γ ’, MC and M_23C_6, and no harmful phase was observed in the alloy. M_23C_6-type carbides discretely distributed nearby grain boundaries as the alloy was aged at above 840 ° C. The microhardness decreased with increasing aging temperature. The coarsening of γ ’led to the increment of microhardness at 780 ° C and 810 ° C for a short aging time, and a signi fi cant decrease in microhardness after aging at 840 ° C. The aging temperature had more signi fi cant role on the microstructure than holding time. Thus, to obtain optimum strengthening effect for this alloy, the aging temperature should not exceed 810 ° C.