High alloying Ni-based powder metallurgy (PM) superalloys show excellent fatigue performance and damage tolerance properties, and good creep resistance at 750 ℃, and are used for advanced gas turbine disks and other hot components. The hot-working window of high alloying PM superalloy is narrow because of its poor workability. The formation of the γ+γ′ microduplex structure during the thermomechanical processing always results in a decrease in flow stress and a promotion of hot plasticity. However, the stability of the γ+γ′ microduplex structure has not been evaluated. The high temperature flow behavior of a Ni-based superalloy FGH98 prepared by hot isostatic pressing has been examined by means of uniaxial compression testing isothermally at 1060, 1105, 1138 and 1165 ℃ and at constant true strain rates between 0.01 and 10 s^-1. The microstructural evolution and instabilities during plastic flow have been studied. Under all testing conditions, the as-hipped material exhibits flow hardening, flow softening and steady-state flow sequentially with the true strain increased. The dynamic recrystallization occurs and the γ+γ′ microduplex structures are generated when steady state flow or highest strains achieved at temperatures below the γ′ solvus. The formation of the γ+γ′ microduplex structures results in a remarkable decrease in grain size and a promotion of hot plasticity. The relationships between steady-state grain sizes and steady-state stresses during deformation and the formation mechanism of the γ+γ′ microduplex structure were analyzed. The possibility of the microstructure controlling during hot working was discussed.