Abstract: The flat butt welding tests of different processes for 6061 aluminum alloy and AZ31B magnesium alloy with thickness of 4 mm were carried out by friction stir welding (FSW). Microstructure of the joints was observed and analyzed by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive spectrometer (EDS), mechanical properties of the joints were tested by electronic universal testing machine. The results show that, there are obvious banded structures in the weld nugget zone (WNZ), which are composed of aluminum slices inserted in the magnesium matrix and IMCs dispersed on the band. The types of IMCs are Al₁₂Mg₁₇ and Al₃Mg₂. Crack nucleation and propagation of aluminum/magnesium dissimilar metal FSW joints occur among the banded structure. Welding technology affects the morphology of banded structure and size and quantity of IMCs. With the increasing of rotational speed (n) or the decreasing of welding speed (v), the banded structure in the joint WNZ are curved, relatively short in length and discontinuously distributed. The banded structure becomes thinner, but the quantity of IMCs increase and the size becomes larger when the rotation speed (n) is too high or the welding speed (v) is too low. The ultimate strength of aluminum/magnesium dissimilar metal FSW joint mainly depends on the morphology of the banded structure and the size and quantity of IMCs.