Resistance spot welding process is strongly related to interfacial contact behaviors. The effects of thermal contact is rarely investigated so far and generally ignored in numerical models. In this work, a parametric FE model, which considers the variation of the surface roughness of the electrodes and workpieces, has been developed to investigate the effects of thermal contact on weld nugget formation. With the parametric model, four cases, e.g. ideal smooth surface, minimal roughness surface and maximum roughness surface for steel sheets and electrodes of as-received condition, and highly rough electrode surface, are investigated. Researches show that when the surface roughness of the electrodes exceeds some limit, the thermal contact conductance will substantially affect the weld nugget formation, therefore, must be considered in numerical models to precisely predict welding process.