Abstract: T-joints of 5083-O (wall plate)/6082-T6 (rib plate) dissimilar aluminum alloys were prepared by single-pass and reversed double-pass friction stir welding, and the microstructure and mechanical properties were studied. The results show that the lap interface of single-pass welded joints consisted of unconnected zone, effectively connected zone and kissing bond zone. After reversed double-pass welding, the kissing bond zone disappeared, and the effectively connected zone was broadened. The single-and double-pass welded joints both fractured in the base metal of the wall plate during T-direction tensile with the similar tensile strength, and fractured at the interface and in the heat affected zone of the rib plate, respectively, during L-direction tensile; moreover the double-pass welded joint had higher tensile strength. The single-pass welded joint yielded during the advancing side bending while brittlely fractured during the retreating side bending; the double-pass welded joint yielded during the advancing and retreating side bending. The microhardness of the single-and double-pass welded joints along the wall plate direction was kept at about 75 HV, and that along the rib plate direction decreased first, then increased and then was stabilized from the thermo-mechanical affected zone to the base metal.