The present modeling of a gas tungsten arc in helium or argon accounts for metal vapor contamination from the weld pool as an anode. The whole region of gas tungsten arc welding, namely, tungsten cathode, arc plasma and weld pool is treated in a unified numerical model. A viscosity approximation is used to express the diffusion coefficient in terms of the viscosities of shielding gas and iron vapor. The time dependent two-dimensional distributions of temperature, velocity and iron vapor concentration are predicted, together with the weld penetration as function of time for a 150 A arc at atmospheric pressure. It is shown that thermal plasma in gas tungsten arc is markedly influenced by iron vapor from the weld pool surface and concentration of the iron vapor into the plasma is dependent on temperature of weld pool surface.