Electric resistance spot welding which is an important process in auto-body assembly is simulated by a 3-D finite element code developed. The main interest in the simulation is the quality of welding and the durability of electrodes, which needs electrothermal analysis for temperature distribution in both electrodes and welded sheets. A finite element formulation is derived for both electric analysis and thermal analysis that are coupled together. The contact resistance between electrodes and sheets is especially treated with artificial interface elements in which the material properties for electric and thermal analysis are artificially imposed for physically reasonable simulation.