TC4( Ti-6Al-4V) titanium alloys exhibit a remarkable combination of high specific strength and good oxidation resistance, which make them useful for aerospace and chemical industrial applications. However, they suffer from low wear resistance, which limits them in applications where abrasive or erosion phenomena are present. Over the last decade, various types of surface modification techniques have been developed aimed at improving the wear resistance of Ti and its alloys. Among the various surface engineering methods, laser surface nitriding is a promising method. In present paper, laser surface nit riding is carried out on the surface of TC4 alloy by Nd:YAG laser in a mixture of nitrogen and argon gases with different ratio. The influence of N2 ration on the microstructure and hardness is investigated．The results show that the nitriding zone is divided into three layers, i.e, melting zone, heat affected zone and base material. Melting zone consists of TiN in a form of dendrites. Dendrite consists of orientated growth columnar grains at the bottom, with a small dendrite dendrites existed in the form of needle-like. There are obvious micro-cracks in the melting zone, which caused by thermal stress. The stress is the result of the surface rapid melting by the laser pulse, and then solidification quenching. The microhardness of the nitriding is approximately 600～800 HV, which is 2 times of the TC4 substrate.