Abstract: Thermal expansion deformation for resistance spot welding of low-carbon steel is simulated by an finite element model. Theoretical thermal expansion deformations for normal welding, or welding under restraint of workpiece and some extreme conditions are provided. The action of workpiece expansion on electrode and the influence of mechanical restraint on expansion behavior are determined. Following conclusions have been drawn: (1) The thermal expansion causes a heavy push on electrode and the maxmium expansion displacement of workpiece is in axial direction. (2) Thermal expansion deformation distribution changes with the variation of workpiece-restrianted condition, electrode force or the mechanical parameters of welding machine. The modeling provides a theoretical guidance for the study of real-time quality control based on thermal expansion.