The microstructure evolution and its effect on the impact toughness of a new Ni-Fe based alloy GH984G, used in 700 ℃ ultra-super critical coal-fired power plant, were investigated during thermal exposure at 650℃-750℃ for up to 10,000 h. The results show that the impact toughness at room temperature drops rapidly at the early stage during thermal exposure at 700 ℃ and then has no significant change even if after exposure for 10,000 h. The significant decline of the impact toughness is attributed to the coarsening of M23C6 carbides at grain boundaries, which weakens the grain boundary strength and leads to the aging-induced grain boundary embrittlement. The M23C6 carbides have almost no change with further thermal exposure and the impact toughness also remains stable. Additionally, the impact toughness rises with the increase of thermal exposure temperature. The size of γ′after thermal exposure at 750 ℃ for 10,000 h is much bigger than that at 650℃ and 700℃ for 10,000 h. Therefore, the intragranular strength decreases significantly due to the transformation of the interaction between γ′ and dislocation from strongly coupled dislocation shearing to Orowan bowing. More plastic deformation occurs within grains after thermal exposure at 750 ℃ for 10,000 h, which increases the impact toughness.