Abstract: The evolution of three different initial microstructures of GH864 alloy after heat treatment was studied by optical microscopy(OM) and field emission scanning electron microscopy(FESEM).The impact toughness at room temperature and crack propagation rate at 650℃ were analyzed after heat treatment.The results indicate that the grain microstructural evolution of GH864 alloy is hereditary between initial forging state and heat treatment state after 1020℃,4h/air cooling→845℃,4h/air cooling→760℃,16h/air cooling,while carbides in grain boundaries and γ' phases are not significantly different.The larger the grain size,the better crack propagation behavior the alloy has,and the worse the impact toughness is.It is highlighting that the necklace microstructure has a better effect to improve the impact toughness and crack propagation behavior of GH864 alloy.