Abstract: The diffusion and deposition composite layers of Mo-N were fabricated on titanium alloy of Ti6Al4V surface by the plasma surface alloying technique. Three kinds of the composite layers were obtained by adjusting ratio of N2 to Ar. The structure, microhardness, resistant corrosion property of composite layers on the surface of alloy were investigated respectively. The results show that the structure and hardness of the diffusion and deposition composite layers on the surface of alloy were affected by ratio of N2 to Ar and the composite layer could increase resistant corrosion of Ti6Al4V alloy in 37wt% boiling HCl solution. The thickness of composite layers raised, hardness increased and corrosion resistance worsened with increase of ratio of N2 to Ar.