Abstract: The surface of 40Cr steel was treated by melting hardening with the axial flow CO2 laser and the transversal flow CO2 laser respectively.The microstructure and depth of melting hardened layers was analyzed by means of scanning electron microscope and microhardeness sclerometer.The results show that the melting hardened layer treated by axis flow and transversal flow CO2 laser both consisted of hardening zone and transient zone.The microstructure of the two zones was mixed martensite + retained austenite,and tempered martensite + retained austenite + ferrite + pearlite + carbide.The depth of the hardened layer treated by these two lasers both increased with the increase of energy density.The depth of hardened layer treated by transversal flow CO2 laser was deeper than that treated by the axis flow CO2 laser.