Microelement Hf added in Ni-based powder metallurgy (PM) superalloy can modify microstructure and improve mechanical properties, such as stress-rupture life, creep resistance and crack growth resistance, and also benefit to eliminate notch sensitivity. So systematically studying the effect of microelement Hf on PM superalloy microctructure will help to comprehend its corresponding mechanism. The effects of microelement Hf on the morphologies, chemical compositions and content of γ’ phase and MC carbide in FGH97 PM Superalloy were investigated by means of SEM and physiochemical phase analysis. The results showed that Hf facilitated the precipitations of γ’ phase and MC carbide, and changed chemical compositions of γ’ phase and MC carbide, the effect of Hf on the size and morphology of MC carbide was not obvious, while Hf greatly affected the size and morphology of γ’ phase and accelerated the splitting of γ’ phase from one instable cubic γ’ particle to stable octet of cubes. As Hf affected the lattice misfit of γ’/γ phase (d), modifying Hf content changed the critical splitting size of γ’ phase (D_c). The relationship between D_c and Hf content (w(Hf)) was found to be D_c=315.4+640.2w(Hf)-358.2[w(Hf)]². With Hf content increased, the absolute value of d decreased and D_c increased. Cubic γ’ particle split into an octet of cubes when γ’ phase grew up to the critical splitting size.