Abstract: Experiments of refill friction spot welding of Mg/Al dissimilar metals were carried out.The mechanical properties and microstructure of the joints were tested and analyzed.The results showed that the weld spots had smooth appearance and high shear strength,and the shear stress could reach 1865 kN at the proper welding parameters if the lap joints were properly designed.Little cavity and microcracks are found on the nugget/Mg vertical interface,where is the fracture zone.At the horizontal nugget/Al interface with certain thickness,where the microhardness is obviously higher than that of two-side materials, which has some relation with the formation of brittle and hard intermetallics.