AZ91D wrought magnesium alloy plates, with thickness of 6.3 mm, are welded by a CO2 laser beam with a laser power of 3500 W, welding speed of 2.5 m/min, defocusing of +2 mm and a constant argon gas flow of 10 L/min. The microstructure, tensile properties and fracture morphology of the joint are investigated. The results show that a favorable weld can be obtained. The fusion zone consists of finer α-Mg equiaxed matrix and β-Mg17Al12 precipitates distributed along the grain boundaries with pores and microcracks frequently found. The width of the partly melted zone is very narrow, about 50～80 μm, and the grains near the fusion zone are locally melted while those near the base metal are not. Tensile results indicate that the ultimate tensile strength (UTS) and elongation of the joint are respectively 232 MPa and 6%, about 90% and 25% of that of the base metal. The final fracture of the joint occurs at the center of the fusion zone and the fracture morphology shows a combination of brittle and ductile features.