The microstructural evolution and mechanical properties of nickel-based heat-resistant alloy C-HRA-2 (equivalent to alloy 617B removing γ' phase) aged at 700 °C for up to 10,000 h were investigated in present work. The effect of nickel-based alloys with or without γ' phase is analyzed and discussed. The experimental results prove that the yield strength of the alloy without γ' phase (460 ± 5 MPa) is only about 25 MPa lower than that with γ' phase (485 ± 5 MPa) after long-term aging. But the impact toughness of the alloy without γ' phase (120 ± 5 J) is twice as much as that with γ' phase (60 ± 10 J). Due to the absence of γ' phase hindrance, a large amount of needle/rod M23C6 could grow in grains of C-HRA-2 alloy during the aging process. The length of these M23C6 needles/rods ranges from 0.1 µm to over 1 µm and the thickness is around 20 nm. They mostly grow along {111} and {022} planes and form a network distribution, hindering dislocations' movement to increase the alloy's strength. The strength contribution of needle/rod M23C6 is considerable, which makes alloy C-HRA-2 have a high yield strength. At the same time, without the existence of γ' phase, the aggregation degree of the immobile dislocations in the alloy is reduced. Thus, the formation of voids and cracks is thought to be greatly delayed, improving the ductility of the alloy.