Abstract: The fracture mechanics theory was used to analyze the two fracture modes in shear tension tests for AZ31B magnesium alloy joint of resistance spot welding.The shear tension test results showed that the fracture mode of the AZ31B magnesium was influenced by the maximum tensile stress at the nugget boundary.The button-pullout fracture will happen if the maximum tensile stress at the nugget boundary is lower than the tensile strength of the base metal;otherwise the interfacial fracture happens.And there is a critical nugget diameter when the failure mode is transferred from button-pullout fracture to interfacial fracture.If the nugget diameter is less than 6.5 mm,the button-pullout fracture will occur,and the interfacial fracture will happen if the nugget diameter is larger than 6.5 mm.The calculation on the equivalent stress intensity factor at the spot weld indicates that the interfacial fracture is harder to occur with the stress intensity factor being increased,and the fracture mode has little influence on the load-capacity of the joint.