In order to develop some fatigue design diagrams, the authors carried out weld joint fatigue tests for an aluminum alloy carbody shell. The hollow extrusion weld joints of a doubled-skinned shell and two types of reinforced weld joints of a single-skinned shell were selected as being representative of carbody shell weld joints. The authors created a fatigue design diagram for the hollow extrusion weld joints by means of a statistical treatment of an S-N curve obtained from the fatigue tests using both small-size and full-scale test specimens. In addition, they used the full-scale test specimens in the same manner to develop a common diagram for two kinds of reinforced welds used on single-skinned shells. The diagrams were valid for evaluating fatigue strength from the stress range of materials near the weld joints.