A shielding gas composition-controlled (SGC) welding process has been developed in which shielding gas composition changes periodically. This welding process has been successfully developed on the basis of computer simulation. A simulation model incorporating periodic change of shielding gas composition has been proposed and verified by welding experiments and observation of wire tip behaviour. In order to change shielding gas composition effectively, CO2 gas is locally and periodically introduced into Ar+2% O-2 shielding gas. As a result, CO2 and MIG arcs occur periodically during welding and the arc pole strongly oscillates over the groove wall. Oscillation control of the arc pole can be achieved by this shielding gas composition-controlled method. This newly developed welding system makes it possible to perform stable welding in narrow V-grooves and excellent welding joints are obtained with good fusion.