Ti-6Al-4V alloy are widely used in chemical and aircraft industries for their good corrosion resistance and high strength to weight ratio. Surface alloying of Ti alloy by $CO_2$ laser is able to produce few hundred micrometers thick TiN surface-alloyed layer with high hardness on the substrate very simplely by injecting reaction gas($N_2$) into a laser-generated melt pool and adjust the hardness to the specific requirements of the individual application by changing of laser processing parameters. This research has been investigated the effect of such parameters on TiN surface-alloying of Ti-6Al-4V alloy by $CO_2$ laser. The maximum hardness of TiN surface-alloyed zone waw obtained by injecting 100% $N_2$ gas and it was decreased as the amount of $N_2$ gas in Ar and $N_2$ gas mixture was decreased. As scanning speed was increased, the hardness and depth of TiN surface-alloyed zone was decreased at constant laser power. The surface hardness after double scanning laser treatment is higher than that of single scanning. At constant laser power, the surface roughness is increased after the surface alloying if laser scanning speed is decreased.