Hf or Ta is widely added to powder metallurgy (PM) Ni-based superalloys to improve their microstructure and mechanical properties. However, research on the mechanism by which Hf and Ta synergistically affect creep performance is lacking. In this study, the effect of Hf and Ta on the creep rupture characteristics and properties of PM Ni-based superalloys at a temperature range of 650-750 ^oC was studied at multiple scales using SEM, EBSD, and TEM. The results showed that Hf and Ta significantly prolonged the creep rupture time, reduced the minimum creep rate, and improved the operating temperature and creep strength. The types of fracture morphologies at various creep temperatures were consistent, the crack source area exhibited intergranular fracture, the crack propagation area exhibited mixed fracture, but the addition of Hf and Ta significantly reduced the fraction of the crack source area. Considering that the stacking fault energy increased with increasing creep temperature, the creep deformation mechanism changed from microtwin shearing at 650 ^oC to microtwin and superlattice stacking fault shearing at 750 ^oC. In addition, Hf and Ta increased the content of MC-type carbides, significantly reducing the density of annealing twin boundaries and effectively inhibiting the nucleation of secondary cracks. Hf and Ta refined the M₂₃C₆ phase on grain boundaries and discontinued its precipitation, thereby strengthening the grain boundary and inhibiting the intergranular fracture. Hf and Ta reduced the stacking fault energy at various creep temperatures and increased the density of deformed microtwins, thereby increasing the resistance to dislocation movement. Moreover, Hf and Ta increased the volume fraction and average size of the secondary γ′ phase and increased the lattice mismatch between the γ and γ′ phases, thereby enhancing the strengthening effect of the γ/γ′ phase interface.