Four powder metallurgy (PM) Ni-based superalloys with different Hf and Ta contents were creep-tested at 650 °C and 970 MPa, 700 °C and 770 MPa, and 750 °C and 580 MPa, respectively. The effect of Hf and Ta on creep deformation behaviors of the superalloys was studied from multiple scales by SEM, electron backscatter diffraction (EBSD), and aberration-corrected scanning transmission electron microscope (AC-STEM). The results showed that Hf and Ta suppressed the intergranular fracture and initiation of cracks during the acceleration creep stage, which prolonged the creep rupture time. Hf and Ta inhibited the stacking faults extending and the dislocation climbing and promoted the Suzuki segregation of W during the steady-state creep stage, which reduced the minimum creep rate and delayed the start time of the acceleration creep stage. The Suzuki segregation of Co, Cr, Mo, Ti, Nb, W, and Ta along stacking faults was observed after Hf and Ta addition, leading to the localized phase transformation in the γ′ phase, and the stacking fault phase was chemically disordered. This study provided ideas for the composition design of novel PM Ni-based superalloys and theoretical foundations for the combined addition of Hf and Ta.