The effect of solution cooling rate on the microstructure and microhardness of a novel type nickel-based P/M superalloy FGH98Ⅰin subslovus plus superslovus and superslovus heat treatment condition was studied by means of optical microscope (OM), field emission scanning electron microscope (FESEM) and microhardness tester. The results show that subslovus pre-heat treatment makes the large grain boundary γ ' phase of as-forged alloy partial dissolution, and the grains grow a little, while the influence on the cooling γ ' precipitates of superslovus heat treatment is not obvious. The average sizes of secondary and tertiary γ ' precipitates decrease with the cooling rate increasing. Meanwhile, the shape of secondary γ ' precipitates changes from the butterfly-like to the spherical, the γ ' phase particle density increases and the area fraction decreases. It is also found that γ ' phases nucleate and precipitate in two stages if the cooling rate is not faster than 1.4 °C/s. The higher the cooling rate, the higher the microhardness, and the microhardness increases more after aging. In addition, the average sizes of γ′ precipitates and the microhardness as a function of cooling rate are established. The above results can be used as a theoretical reference for the selection of actual superslovus processing of FGH98Ⅰdual-property disk