The lattice-structure of nanocrystalline pure Ni prepared by high-frequency sputtering was investigated. The lattice parameter of nanocrystalline Ni samples with grain sizes ranging from 6 to 15 nm was measured accurately by X-ray diffraction. It was found that the lattice parameter (a) for the nanocrystalline samples is larger than that for the perfect Ni single crystal (a 0 ), and the value of a increases significantly with the decrease of grain size. According to the thermodynamic theory, the lattice expansion in the nanocrystalline sample was explained.