Abstract: 5083 aluminum alloy and AZ31 magnesium alloy were lap jointed by using friction stir lap welding.The mechanical properties, microstructure and fracture appearance of the joint were investigated.The results indicated that a high shear strength, interface bonding and defect-free weld have been obtained when the probe of a tool did not contact the lower Mg sheet and the suitable welding parameters are used.5083 and AZ31 were jointed by a conversion layer that existed in interface zone which contained intermetallic compounds.The joints fractured in interface.Fractographs confirm that the fracture of the Al side is composed of the "small pit" by plastic tearing, but the fracture of the Mg side consists of river line pattern mainly.The micro hardness raised in the interface zone.