By means of surface mechanical attrition treatment (SMAT), nanocrystalline surface layers are produced in pure Ni plates. The average crystallite size, root mean square (r.m.s.) microstrain, dislocation density, and stored elastic energy are determined by X-ray diffraction (XRD) line profile analysis. The average crystallite size obtained by XRD is compared with the grain size observed from transmission electron microscopy (TEM) image. The high-resolution TEM (HRTEM) micrograph confirms the presence of high density of dislocations obtained by XRD, and reveals that most of dislocations distribute at the subgrain boundaries with few inside the subgrains.