This work aims to study the differences in the microstructure and mechanical properties of a new nickelbased alloy without γ' phase during long-term aging at 675 ℃. The test alloy C-HRA-2 for nextgeneration power plant applications was simulated for 5000 h at the service temperature. The formation of long needle-like M23C6 carbides inside the grains is a unique phenomenon compared to conventional nickel-based alloys. After aging for 500 h, the microstructure, strength and hardness tend to be stable, while the toughness shows a slow downward trend. Even so, the toughness still exceeds 110 J after aging for 5000 h. Compared with the alloy 617/617B containing the γ' phase, the strength of the alloy without γ' phase (C-HRA-2) reduced by about 5e10%, and the hardness lowered by 5e15%, but the elongation also increased by about 10e50%, and the toughness increased by about two times.