Friction stir welding (FSW) has been used for joining AA5083 and AA7B04 alloy sheets with the aim of studying the microstructure and the mechanical properties of dissimilar FSW joints obtained by varying the initial base metal state of AA7B04 alloy. The results show that the initial base metal state has a significant impact on the material flow during dissimilar FSW. As compared with the joints placing hard alloy (artificially aged AA7B04-AA or naturally aged AA7B04-NA) on the retreating side, it becomes easier transporting AA5083 from advancing side to retreating side when soft alloy (annealed AA7B04-O) is placed on the retreating side. The atomic diffusion does not occur at the interface between AA5083 and AA7B04, indicating that the mixing of the two materials is merely mechanical. Grain refinement is observed in the stir zone. The failure location during tensile tests is different depending on the initial base metal state. The joints (AA5083/AA7B04-AA and AA5083/AA7B04-O) fail in the base metal on the soft material side which corresponds to the minimum values in hardness profiles. Differently, the joints (AA5083/AA5083 and AA5083/AA7B04-O) fail in the stir zone due to the presence of defects including ¿zigzag line,¿ kissing bond and discontinuous voids.