Recently, aluminium alloy has increasing been adopted to the welded structure, the more widely use of the higher-strength work-hardened Al-Mg alloy (such as 5083-1/4 H or 1/2 H) would contribute to the weight-reducton of the structures. One of the most important problems in the adoption of these alloy to the welded structures is the reduction of joint efficiency due to the softening of heat affected zone. The width of softened zone of the welded work-hardened alloy varies with welding conditions it is reduced by using small welding heat. From another point of view, the knowledge of the characteristics of these softened zone, would make it possible to cover the decrease of strength due to softened zone by using suitable joint design. In this report the author made clear the followings; (1) The fundamental nature of welded joints with softened zone by inquiring after the factors most connected with softened of welds and the relation between the width of softened zone and welding heat or tensile strength of welded joint of the work-hardened aluminium alloy, as well as (2) The possibility of increasing the strength of structural member including weld softened zone by adopting the appropriate joint design, from the results of tensile tests on longitudinal transverse and combined of shifted cross joints, and the stress analyses there of.