In this paper, the Gas Metal Arc Welding (GMAW) process is studied and temperature field is gained in this process. Thermal effects of Gas Metal Arc (GMA) and temperature field from it on workpiece (copper) and shielding gas type, is the main key of process optimization for GMAW. Energy source properties of GMA strongly depend on physical property of a shielding gas. In this paper, carbon dioxide (CO2) was used as an alternative gas for its low cost. The basic energy source properties of CO2 GMA were numerically predicted ignoring the oxidation of the electrodes. It was predicted that CO2 GMA would have excellent energy source properties comparable to that of He, Ar GMA. The numerical results show the time-dependant distributions of arc pressure, current density, and heat transfer at the workpiece surface are different from presumed Gaussian distributions in previous models.