Abstract: AZ31 magnesium alloy and 6061 aluminum alloy were butt welded by friction stir welding at different welding speeds (23.5, 47.5, 75.0 mm·min^-1). The effects of welding speed on the microstructure and mechanical properties of the joint were investigated. The results show that stripped structures existed in the nugget zone of the joints welded at different speeds. The grains in nugget zone were refined. A small amount of Al₃Mg₂ and Al₁₂Mg₁₇ metallic compounds formed at interfaces between nugget zone and thermomechanically affected zone on advancing side. With decreasing welding speed, the grain size and the material mixing uniformity in different zones both increased slightly. With increasing welding speed, the hardness in different zones on retreating side decreased first and then increased, while that on advancing side showed an increase trend; the tesile strength and yield strength of the joint both decreased. The joint fractured at the interface between nugget zone and thermomechanically affected zone on advancing side. The fracture mode was brittle.