The purpose of this study was to investigate the resistance of spot welding to DP590 and SPFC440 steels, optimizing the welding parameters and investigating the microstructure and mechanical properties of the joint. The resistance spot welding process due to its high speed and the ability to produce spot welds automatically becomes the main process in the welding of a car body. DP590 steel with martensitic ferritic microstructure and desirable mechanical properties are among the advanced high strength steels as SPFC440 bonded steel and selected perlitic ferritic microstructure with excellent mechanical properties and formability. The welding current parameters from 8KA to 17KA and welding times equal to 10Cycle, 15Cycle, 20Cycle and 25Cycle were considered as variable and applied electrodes were fixed parameter equal to 6.2KN. The shear tensile test was used to investigate the mechanical properties of the joint and the mechanical properties including strength, elongation and fracture energy of the spot welds were obtained. Structural study of the spot welds was done by stereography as well as light and electron microscopes and a hardness profile was prepared using a micro hardness tester. The effect of welding parameters on the geometry and appearance of the joints as well as their fracture modes were investigated by measuring the diameter of the spot welds and their fracturing. In this study, the microstructure of the weld zone (FZ) was mainly martensitic due to the high cooling rate in this welding process. The microstructure of the heat-affected zones consisted of three zones, in which the transition from weld metal to base metals decreased the size and size of martensites and increased to the base metal phases. The hardness of the weld metal was equal to 560HVN and higher than the base metal hardness and the highest hardness belonged to the coarse-grained heat-affected zone on the base metal DP590 and 612HVN. At low inlet temperatures, interfacial fracture (IF) was observed, and at higher inlet temperatures, pullout fracture (PF), and environmental fractures started from the intracritical heat-affected zone on the SPFC440 base metal, which had the lowest hardness. As the welding current and welding time increased, the mechanical properties and diameter of the spot welds increased, with the highest tensile strength equal to 27800KN and with an increase in the heat input temperature of the melt spraying or extrusion phenomenon. For welding time 20Cycle in 17KA current and for welding time 25Cycle in 16KA current melt spraying and loss of mechanical properties occurred. Finally, due to the diameter of the spot welds, the failure modes, and the mechanical properties of the joints, the permissible welding parameters for DP590 / SPFC440 asymmetric joints were set in the form of a lobe diagram Keywords: Resistance Spot Welding, DP590 Dualphase steel, SPFC440 high strength steel, mechanical properties of spot welding, microstructure of welding zone, heat affected zone