Abstract: This paper systematically analyze the effect of external magnetic field (EMF) generated by permanent magnetic on the nugget size, mechanical properties and failure mode of aluminum alloy resistance spot welds. The relationship between nugget size and peak load was also studied. The results show that the work distance between two permanent magnets has significant effect on the nugget diameter. The smaller the distance is, the larger the nugget diameter is. The nugget diameter and tensile-shear force were increased under the help of EMF. Under different welding parameters, the nugget diameter can be increased by 3.5%~14.1%, and the tensile-shear force enhanced 4~25%. The EMF promotes the failure mode change from interfacial failure to pullout failure. The EMF can improve the strength-toughness properties and refine grain size. The relationship between peak load and the product of nugget diameter, workpiece thickness and base material ultimate tensile strength shows well linear relation.