Abstract: The nugget formation, microstructure and mechanical property were studied for the resistance spot welding of AZ31B magnesium alloy. The results showed that a large welding current could increase the diameter, the penetration and the effective load area of the nugget observably. The microstructures evolution from the base metal to the nugget center was flat grain—cellular grain—cellular dendritic grain—equiaxed dendritic grain. The equiaxed dendritic grains were maily distributed in the nugget center, among the grains the strengthen phases were distributed. Softening phenomenon was occurred on the edge of the nugget as the welding current was large, and the hardness inside the nugget was lower slightly. On the other hand, a large welding current could increase the effective load strength of the spot weld, which obviously increased the welding quality.