Experimental results on the welding of low alloying steel (18CrMnTi) and 45 steel with 3-5kW CO2 laser are discussed. The depth/width ratio being about 2, the maxium penetration depth is about 5.5mm. Various technological parameters influencing the effect of laser deep penetration are studied and some effective measures for overcoming the defect (crack, gas hole) of aser deep penetration welding have been found.