The solid-state friction stir processing (FSP) was introduced to surface nitriding on Ti-6Al-4V substrate under nitrogen atmosphere. Under the optimized process conditions, a sound and thick nitriding layer was successfully prepared and detailed by XRD, SEM and EDS methods. The outer thin layer on the top surface crowns was mainly TiN phase with refined grains in size of 2-3 mu m. After removing the crowns, a more than 150 mu m thick nitriding layer was remained with a composite-like structure. The transition layer contained numerous fine and uniformly dispersive nitride particles embedded in the Ti matrix with a certain solid solution of Ti(N). The micro-hardness of the top surface was similar to 1105 HV0.2. Furthermore, the formation mechanisms including rapid thermalchemical reaction, nitrogen diffusion and nitride particle dispersion were discussed in detail. (C) 2013 Elsevier B.V. All rights reserved.