Abstract: The resistance seam welding (RSW) test of DP780 Dual Phase Steel was carried out without preheating. The microstructure and mechanical properties of the welded joint were investigated by SEM, cupping tests and mechanical properties tests. The results show that the microstructure of the weld zone mainly consists of granular martensite. The microstructure of heat affected zone is mainly ferrite and martensite. DP780 Dual Phase Steel joint has good mechanical properties. The hardness of the weld zone is higher than that of base material and the heat affected zone of the joint. The tensile tests show the joint fractures at the base material, indicating that the joint strength exceeds that of the base material. However, the cupping test reveals an insufficient plastic reserve in the joint.