Abstract: This paper deals both with fundamental mechanism and joint performance of undermatching weldments.Static tensile strength of undermatching welded joint is influenced by relative thickness of the welded metal.As the relative thickness decreases, the joint tensile strength increases from that of the weld metal to the base metal strength.The joint strength also increases with the plate width of the undermatching welded joint,until the width becomes larger than about five times as the platethickness. Ultimate tensile strength of the undermatching welded joint can be well estimated by a formula including variables such as the relalive thickness and the strength ratio of welded metal vs.base metal.