Gradient hierarchical structures, including grain size gradient, twin density gradient, and texture gradient, are believed to broke up the long-standing dilemma-"strength-ductility trade-off' in materials science. Here we study the effect of dislocation density gradient on deformation behavior of pure nickel subjected to torsion. After applying torsion to cylindrical pure nickel samples, we find that Vickers hardness gradually decreased from surface to centre and the there is no obvious grain size gradient along the radial direction. The compression tests have been conducted on samples with different thickness of gradient layers. The results indicate that the yielding strength of the material can be three-times improved. Especially, the work-hardening rate "up-turn" phenomenon which was usually found in gradient nano-structures is presented in the torsion deformed pure nickel. Finally, the possible reasons and the effect of thickness of gradient structure are carefully discussed.