Abstract: To enhance the load-carrying capacity of undermatching joints of high strength steels,the concept of equal load-carrying capacity(ELCC),i.e.weld bead of undermatching weld joints and base metal have equal static load-carrying capacity and fatigue resistance,is presented.Based on security consideration,the welded structures must have sufficient strength and stress relaxation ability,and the type of fracture mode of mismatching weld joints will reflect the welding joint strength,plastic deformation ability and the degree of non-uniform of plastic deformation between weld bead and base metal.Because the most safe fracture model is that the general yield fracture of plastic deformation in the base metal will occur prior to that in the weld bead,the structural design thinking of ELCC is the general yielding guideline as criterion.Based on determination of the realization conditions of ELCC,the weld shape design method of ELCC for undermatching weld joints is also presented.The vulnerable area of undermatching weld joints can be transferred from weld bead to base metal by the ELCC design of joints,so the welded structure can be designed direct in the light of the strength of base metal without regard to the influence of mis-match ratio.