Turbine discs, manufactured using powder metallurgy nickel-based superalloys, serve as critical hot-end components in aviation engines. Considering that the disc rim and hub has to function under different temperatures and stresses, their dual microstructure had been paid close attention. In this study, the coarse- and fine-grained microstructures were obtained by controlling the solution treatment temperature, and the creep behavior of the superalloy at 700 ^oC and under various stresses was investigated. The effect of stress on the creep deformation mechanism and fracture behavior of the alloy was investigated via SEM and TEM. In the coarse-grained microstructure, the creep deformation mechanism at 780 MPa was primarily isolated by stacking faults and microtwin shearing, while the stress increased to 900 MPa, the extended stacking fault shearing and microtwinning jointly dominated creep deformation. Nevertheless, within the stress range of 780-900 MPa, the creep deformation mechanism remained consistent in the fine-grained structure, which was characterized by the coexistence of extended stacking fault shearing and microtwinning. In addition, this study indicated that the grain boundaries exhibited a diminishing promotion effect on the minimum creep rate as the applied stress increased for both grain microstructures. The high stress sensitivity of the experimental alloy resulted in the occurrence of twinning with elevated stress levels. This phenomenon accelerated plastic deformation, resulting in an increased creep rate. Moreover, the creep fracture source zone was predominantly an intergranular fracture, whereas the propagation region was predominantly a transgranular fracture in both grain microstructures. The tendency for intergranular fracture in coarse-grained microstructures decreased with the increase in the creep stress level, vice versa was observed in fine-grained microstructures.