The 3 mm thick AZ31B magnesium alloy butt joints are carried out by fiber laser welding. Weld morphology, microstructural and mechanical characteristics of welded joints are investigated by means of optical microscope, scanning electron microscopy (SEM), tensile test machine and Vickers hardness tester. The results show that the weld zone width is narrow and weld appearances are smooth, but at the end of weld, the top surface collapse and back surface appear concave a little. The weld zone is characterized by fine equiaxed, and heat affected zone microstructures coarsen compared with the basemetal. The weld zone grain size increases according to the weld direction. The hardness in heat affected zone of welded joints is minimum, so the tensile samples without pore and crack defect rupture with ductile-brittle mixed mode in the heat affected zone. The ultimate tensile strength and elongation of is up to 95.1% and 89.3% of the basemetal.