Abstract: The YAG laser was used to weld AZ31B Mg alloy.The microstrueture,element profile,fracture surface and hardness were investigated by the metal phase microscopy,electron probe microanaiysis,scanning electron microscope,X-ray and hardness instrument.The results show that favorable joint can be obtained,which heat-affected zone is notevident,and weld is narrow and the grains of fusion zone (FZ) are fineequlaxed-grain.Besides,element segregation and Mgl7 All2 phase cannot be found in FZ.Vaporizing of Mg element led to the increasing of A1content,but Zn content is constant.At the same time,the Mg element lossincreases at direction of fusion depth.The fracture shows quasi-cleavage,which does not exist porosity and crack.The hardness increases at fusion depth direction.