A model was developed to simulate the temperature distribution and nugget formation during resistance spot welding ( RSW) of zinc-coated steels. It employs a coupled thermal-electrical-mechanical analysis simulating the dynamic RSW process. Temperature-dependent thermal-electrical-mechanical material properties were considered including contact-resistance.The contact area was determined from a coupled thermal-mechanical analysis. A layer of transition elements was used to represent the change of contact area by killing or activating elements. The heat generation and temperature field were computed in a coupled thermal-electrical model. All these analyses were solved using the commercial finite element method ( FEM) based on ANSYS code, and some advanced functions were used by writing a paragraph of codes by the authors.Compared with the results from only coupled thermal-electrical model in which contact area was uniform during the whole process, the result matches better to the experimental results.