Gas Tungsten Arc (GTA) welding with a constricted nozzle is one of new higher-level welding processes. In this process, an arc plasma is strongly constricted due to a gas flow control using an additional gas nozzle which is called “constricted nozzle”. This welding process overcomes disadvantages of a conventional GTA welding such as low welding speed and low welding efficiency. As a result, it achieves a welding of butt joint of quite thin-sheet metals which is difficult for the conventional GTA welding. However the mechanism of this improvement is difficult to be understood by experimental approaches and is still unclear because the arc phenomena are quite complicated. In this study, we developed the computational model which uses a two-dimensional axisymmetric geometry in order to analyse the arc phenomena during the GTA welding with a constricted nozzle. The current density was concentrated to the center part of the arc plasma because the high speed gas which flows along the electrode cools the edge of arc plasma. As a result, the velocity of the plasma flow was greatly increased and the temperature of the arc plasma increased in the GTA welding with a constricted nozzle. Because of the arc constriction, the heat flux to the anode surface greatly increased at the center area. It was clarified that the constricted nozzle achieves the welding of butt joint of quite thin-sheet metals because it offers high energy density arc plasma by the arc constriction.