Abstract: The W-Yand Si-surface alloying layer was formed on 45 steel substrate with a 1.5 kW CW CO₂ laser. The effect of laser surface alloying processing parameters (laser power, beam diameter, travel speed, hight of alloying powder deposit and protective agent) on surface smoothness, hardness and microstructure of laser surface alloying layer was studied. The processing parameters were optimized by experiments. The formation of LSA layer structure with three different sublayers and structure of heat affected zone were interpreted.