Ni-based superalloys have shown great application potential as component materials in aircraft engines because of their excellent mechanical properties at high temperatures. With the development of engine and power plant boiler tubes, the high-temperature creep resistance of nickel-based superalloys has become an important indicator for evaluating the mechanical properties of superalloys. In this study, the creep behavior of the as-cast Ni-based superalloy with the coprecipitation of γ′ and γ″ phases at 750 ^oC and 120 MPa was investigated. The results show that the creep deformation behavior and creep property change with the size of the γ'/γ'' phases. A number of dislocations are cut into the γ'/γ'' phases, forming continuous stacking faults in the γ channel and γ'/γ'' phases when a high amount of compact γ'/γ'' phases are precipitated, leading a inferior creep property. Increasing the size of the γ'/γ'' phases, the dislocations are easily cut in the γ′ phase and isolated stacking faults are formed in the γ′ phase, which significantly enhances the creep property. Further increasing the size of the γ′ phase, the dislocations are piled up on the interface of the γ/γ′ phases, and the γ′ phase is looped with dislocations, which decreases the creep property. Given the precipitation of the deleterious Laves phases, the grain boundaries (GBs) are weakened. However, the stretch of cracks is restrained, and the creep properties of the alloy are enhanced because of the moderate needle-like η/δ phases in the GBs. The precipitation of the overdose η/δ phases provides a favorable location for crack nucleation and accelerates alloy failure.