Laser transmission welding of thermoplastics is gaining importance in industrial series production because of its advantageous properties and the increasing interest in this technology. At the same time, the demand on ongoing investigations and research to understand the processes involved is being developed intensively. In this report, a simplified mathematic–physical model of laser transmission welding based on finite-elements method will be presented. For the first calculations, the material PA6 and the quasi-simultaneous laser welding process mode were chosen. The model comprises of the complete laser welding process, including the heating and the cooling phase. Boundary conditions and relevant process parameters were specified for the simulation, such as the laser beam intensity, the joining pressure, and the welding time. Flow and temperature profiles were then calculated. Because of the array of available boundary conditions, it is possible to continuously improve the model while comparing the simulated data with that obtained in the experiments. The experimental data were gathered by detecting the displacement of tracer particles in dependence on time and place. Moreover, the melt layer thickness was measured. In general, very good agreement was achieved between the calculated and the measured results. Once the steady–state conditions were achieved, no change in the remaining melt layer thickness, temperature, flow velocity, or weld strength was observed. It was seen that the maximum temperature was placed in the upper layers of the absorbent partner and not in the joining surface. Accordingly, the flow behavior is first detected in the absorbent partner, and afterwards in the transparent one. POLYM. ENG. SCI. 46:1565–1575, 2006. © 2006 Society of Plastics Engineers.