Abstract: AZ31 magnesium alloy and 6061 aluminum alloy plates with the thickness of 6 mm were butt-welded by friction stir welding（FSW） technology. The microstructure of dissimilar metal butt joint was analyzed by means of optical microscope（OM）, scanning electron microscope（SEM） and energy spectrometer（EDS）. The mechanical properties were tested by using the universal testing machine. The results show that poor metallurgical bonding is found at aluminum alloy side due to the poor liquidity of aluminum alloy during friction stir welding process. A large number of banded structures with thickness of 1-3 μm are found in the weld center. The energy spectrum and magnesium-aluminum binary phase diagram analysis shows that the banded structures are hybrid structure of δ-Mg, α-Al and Al₁₂Mg₁₇. The tensile strength and elongation of the magnesium-aluminum dissimilar metal FSW joint is 177 MPa and 1.2%, respectively. The mechanical performance of the joint is mainly depends on plastic flow law of metals and the distribution of the intermetallic compound layer.