Ni nanopowders with various grain sizes were successfully prepared by anodic arc discharging plasma method.The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and the corresponding selected-area electron diffraction (SAED) to determine the crystal structure, lattice parameter, morphology and particle size. The experiment results indicate that the samples have fcc crystal structure as the bulk materials. The lattice parameter of Ni nanopowders is always larger than the equilibrium value of the perfect single crystal lattice, so the lattice expansion has taken place. The lattice parameter and the value of the unit cell volume increase significantly with the decrease of the grain size, and the increase in lattice parameter is inversely proportional to the grain size. The lattice expansion is the result of the interfacial energy and surface tension induced mutual attraction of Ni grains, and this phenomenon can be explained according to the thermodynamic theory.