Abstract: The friction stir welding of MBL10-200 Mg-Li alloy with a thickness of 3 mm was investigated. Concentrating on the process characteristics, the effects of friction tools, welding parameters on weld appearance, and mechanical properties of the joints were studied. The mechanism and solution of joint ductility loss were both investigated. The results show that with relatively wide process window (rotation speed 800~1 600 r/min, welding speed 200~500 mm/min), welds with good appearance can be obtained. Joints under most welding parameters fracture at base metal, and shows comparable strength with base metal. The joint elongation shows apparent decrease, 8%~57% of base metal under varied welding parameters. The non-uniform deformation of WNZ, TMAZ and base metal in the joint is the main reason of ductility loss. Annealing of the welded joints can effectively increase the elongation, nearly 96% of base metal.