Dissimilar metals laser welding technique was developed by using solid/liquid state reaction at the interface. However, intermetallic phase (IMP) formed at the interface of a dissimilar metals joint reduces strength and reliability due to brittleness and weakness of IMP. In the present study, a controlling technique for IMP formation at the interface of steel/aluminum alloy dissimilar metals laser lap joint was studied. Effect of applying a contact block (backing plate) on weldability and strength of the joint was investigated in FEM analysis as well as welding experiment.