Abstract: The microstructure of as-cast GH625 alloy was investigated on the basis of experimental methods such as Thermo-Calc software,scanning electron microscopy and energy dispersive spectroscopy. The effect of casting temperature on the microstructure was also analyzed by comparing different samples. The microstructure of as-cast GH625 alloy is a dendritic structure and M₆C carbides accompanied with δ phase precipitate in the interdendritic area. Both primary dendrite arm spacing(λ₁) and second dendrite arm spacing(λ₂) increase with the increase of casting temperature; however,the microsegregation is affected by both diffusion distance and diffusion time. The comprehensive interaction of these two factors leads to the greatest microsegregation degree of Nb at 1420℃,which contributes to δ phase precipitation in the interdendritic area. As a result,diffusion distance and diffusion time should be comprehensively considered in making the casting process,so as to choose a proper casting temperature(or cooling rate).