Low carbon steel is difficult to be hardened by normal method of heat treatment because of its low content of carbon.According to the character of laser heat treatment,we try to harden low carbon steel by laser heat treatment.In this paper we can obtain the hardened layer of microhardness HV500 and depth 0.5mm after laser heat treating of low carbon steel containing 0.2% C.Metallographic test results show that the microstructure of hardened layer is lathy martensite.We assume that the ultrahigh hardness is mainly due to the ultrafine grains,high density dislocation and more content of carbon in martensite.