Supersonic Fine Particle Bombarding (SFPB) was performed on Ti-6Al-4V alloy to form surface nanostructures. The effect of the surface nanostructures on the gas nitriding was investigated by varying the nitriding temperature and time. Through SFPB process, a nanocrystalline surface layer with a mean grain size of 16nm was obtained, presenting a high hardness of 640 HV. In comparison to the surface without SFPB treatment, the treated surface exhibited a thick nitrogen diffusion layer and high hardness under the same nitriding process. (C) 2013 Elsevier B.V. All rights reserved.