This paper presents laser beam welding (LBW) of an age-hardened aluminum alloy and a bake-hardening (BH) steel, which are promising materials for lightweight car body, compared with conventional arc welding. The LBW of the aluminum alloy had a significantly narrower fusion zone and heat affected zone (HAZ) than the arc weld and the hardness of the softened zones was almost completely recovered to the base metal hardness after a post weld aging treatment. This advantage of LBW is due to a low overall heat input resulting from its high energy density. This is also beneficial for welding of the BH steel. In the LBW of the pre-strained BH steel, both the fusion zone and HAZ were bake-hardened after a heat-treatment and the fracture occurred in the base metal in the tensile test. In contrast to this, the arc weld had a large softened zone in the HAZ and fractures in this region.