Compared with ordinary coarse-grained metals, the nanocrystalline metals exhibit higher strength and hardness, but have a decrease in ductility. To investigate the reasons of low ductility in nanocrystalline metals, contrastive study of mechanical behaviors between high purity, fully dense coarse-grained Ni sheets and nanocrystalline Ni sheets was carried out under uniaxial quasi-static tensile loading through digital image correlation method in this paper. From the displacement and strain fields of the two sorts of samples, uniform plastic deformation was observed in the coarse-grained Ni sample. Whereas, the nanocrystalline Ni exhibited shear localization in the form of shear band, which is the main reason of the early failure during plastic deformation in nanocrystalline Ni.