This study is focusing on understanding the thermal stress formation in hybrid laser-MIG welding which has been gaining a lot of interests due to its many advantages over laser welding. Thermal stress formation is tightly associated with heat and mass transfer in hybrid laser welding, so accurate analysis of heat transfer process in the welding process is critical to correctly predict thermal stress information and residual stress in hybrid laser welds. In this study, a comprehensive heat and mass transfer model analyzing the energy, mass, and momentum transport processes in hybrid laser-MIG welding is successfully integrated with a mechanical model to study thermal stress formation in hybrid laser-MIG welding and residual stress in final welds as well. High compressive stress is found to exist on the top surface of the weld which might cause irregular welds topology and high tensile residual stresses were found in some locations in the final weld. This proposed study can be used as a foundation to further understand the thermal stress formation mechanisms in welding and to provide an efficient way to optimize the hybrid laser-arc welding process.