In this paper, the high-temperature tensile tests were carried out on the millimeter-level coarse-grained new Ni-Cr-Co-based alloy with different γ′ phase sizes at 600 °C and 5 × 10_{^{? 4}}s_{^{? 1}}, and the effects of γ′ phase sizes on the high-temperature tensile behavior and sawtooth rheological effect of the alloy were studied by OM, SEM, and TEM. The results show that the size of γ′ phase has a significant effect on the mechanical properties of the alloy. With increasing of the size of γ′ phase, the material’s strength increases at first and then decreases. The main deformation mechanism of the alloy changes from dislocation cutting through γ′ phase to dislocation by passing γ′ phase. When the size of γ′ phase continues to increase, it is difficult for the dislocation movement to be blocked to by pass the γ′ phase. The solute atoms can pin the movable dislocation. When the stress increases to a certain extent, the dislocation is depinning. Repeated pinning and depinning, that is to say, the dynamic strain aging leads to the sawtooth flow phenomenon in the deformation process of the alloy, which can be weakened by adjusting the size of γ′ phase. When the average size of γ′ phase is 57.18 nm, the serrated flow effect is weak, the critical strain is the largest, and the mechanical properties became higher. Therefore, the optimal size of the γ′ phase is 57.18 nm.