New generation steels have double strength and higher toughness than the normal ferrite steels because of their micrometer or sub-micrometer sized grains. The weldability of the steels is the key for the success of the steels. In this paper SS400 steel is welded by CO2 laser beam and welded joint with higher toughness and strength than the base metal is obtained. When the microstructures of weld metal and coarse-grained HAZ mainly consist of lower bainite the toughness is the highest. When ferrite side plates occur or the microstructures contain high percentage of martensite. the toughness decreases , but is still higher than that of base metal. The hardness of welded joint is lower than Hv330 and there is no softened zone. The welded joint has good bending ductility and its tensile strength is higher than that of base metal.