By means of X-ray diffraction, γ′ phase and γ matrix lattice parameters and γ/γ′ misfit were measured in FGH97 PM superalloy with different hafnium contents. The results show that the lattice parameter of γ′ phase is smaller than that of γ matrix, and also the value of γ/γ′ misfit is negative. Hafnium mainly exists in γ' phase, the content of hafnium partitioning to γ' phase increases with increasing of hafnium contents in FGH97 PM superalloy, resulting in lattice parameter increasing and decreasing of the absolute value of γ/γ′ misfit.