Abstract: Distributions of magnetic field, both external and induced, in nugget region of resistance spot welding (RSW) under permanent magnets were investigated using numerical simulation method. Based on the complex magnetic force distribution, the fluid flow behavior in RSW was systematically analyzed. The results show that the induced magnetic field predominates in RSW process under the permanent magnets. The complex magnetic flux density gradually increases from the center to the edge of the weld nugget, while the radial magnetic flux density gradually decreases with the increasing working distance of permanent magnets. Two perpendicular magnetic component forces exist in the nugget. One acts in the axisymmetric plane of the electrodes generated by the induced magnetic field, the other acts vertically to the plane generated by the external magnetic field. Driven by the complex magnetic force, the molten metal in the nugget would flow with high velocity in both the axisymmetric plane of the electrodes and the circumferential direction perpendicular to the plane.