The effects of local electrical contact resistance on transport variables, cooling rate, solute distribution, and nugget shape after solidification responsible for microstructure of the fusion zone during resistance spot welding are realistically and systematically investigated. The model accounts for electromagnetic force, heat generation and contact resistances at the faying surface and electrode-workpiece interfaces and bulk resistance in workpieces. Contact resistances are composed of film and constriction resistances, as functions of hardness, temperature, electrode force and surface condition. The computed results show that the bulk dynamic electrical resistance cannot reliably reflect transport processes and nugget shape, unless the local constriction resistance and electric current density are known. Regardless of high film resistance, nugget growth and transport processes are independent of film resistance due to delayed response time of local electric current in the early stage. A decrease in constriction resistance, however, delays nugget formation, enhances convection and solute mixing, and changes circulation direction of the stronger convection cell during cooling period. (C) 2012 Elsevier Ltd. All rights reserved.