The welding property of Mg-9.8Li-2.9Al-Zn alloy with the thickness of 2 mm using the CO2 laser is investigated. The joint forming, weld defects and structural evolution are studied with optical microscope (OM) and scanning electron microscope (SEM). The mechanical property of the weldment is tested using the microhardness and longitudinal tensile test at room temperature. The result indicates that the rapid cooling during the laser welding results in the fine microstructures with more α-Mg phase in the weld, compared with that in the base metal. The metastable phase MgLi2Al transforms into the stable phase AlLi. The weld microhardness (about 105 HV) is bigger than that of the base metal (about 85 HV). The longitudinal tensile test of the weldment shows the fractured location along the base metal with the reference yield strength of 172 MPa and the percentage elongation after fracture of about 24%.