The effects of solution cooling rate on γ′ precipitation and microhardness of a novel nickel-based powder metallurgy (P/M) superalloy FGH98Ⅰin superslovus and superslovus plus subslovus post treatment condition were studied by means of field emission scanning electron microscope (FESEM) and microhardness tester. The results show that the average sizes of secondary and tertiary γ' precipitates decrease with increasing of the cooling rate. Meanwhile, the shape of secondary γ' precipitates changes from the butterfly-like to the spherical, the shape factor and particle density increase, while the area fraction and the apparent width of the grain boundary decrease. 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 faster the cooling rate is, the higher the hardness is, and the more the hardness increases are after aging. Subslovus post-heat treatment makes the cooling γ' phases coarser and more cubic, decreases the shape factor, eliminates the γ′ precipitates-riched grain boundary and lowers the microhardness. In addition, the average sizes of γ′ phases and the hardness 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.