The effects of Hf doping on the phase constitution and its evolution of the SmCo7-x Hf (x) (0 a parts per thousand currency sign x a parts per thousand currency sign 0.5) nanocrystalline alloys were investigated. It was found that a single TbCu7-type metastable phase can be stabilized in the nanocrystalline SmCo7-x Hf (x) alloys with x a parts per thousand currency sign 0.25, and the phase decomposition occurs at higher Hf doping contents. The Hf doping plays a significant role in inhibiting grain growth at high temperatures, which is favorable for the high-temperature magnetic properties. The metastable phase is stabilized by the combined effects of nanostructuring and Hf doping. Moreover, the magnetic performance is increased due to the pinning effect and the high magnetocrystalline anisotropy of the stabilized TbCu7-type phase.