To acquire the boundary of the heat treatment parameters satisfying property boundary is significant for property optimization of powder metallurgy FGH96 turbine disks, especially the boundary of the minimum cooling rate required for FGH96 alloy with a certain grain size under the premise of satisfying the performance requirements. Tensile behavior at 25 ℃, 650 ℃ and 750 ℃ of FGH96 specimens was investigated with different grain size under various cooling rates from solution temperature. The microstructures were observed. The results show that a multi-mechanistic strengthening model including the effect of chemical composition, γ′ size and fraction and grain size agree well with the experimental measurements. The strengthening model was applied to study the minimum cooling rate and grain size. The minimum cooling rate and grain size were obtained as: grain size should be smaller than 14 mm if cooling rates is 55 ℃/min, grain size should be smaller than 16.8 mm if cooling rates is 72 ℃/min, and grain size should be smaller than 20 mm if cooling rates is 81℃/min. The minimum cooling rate and grain size can be used as a guideline to design heat treatment process through simulation.