A gas metal arc welding phenomena simulator was developed, which simultaneously computed an arc plasma behavior and a weld pool formation process with the time evolution by alternately conducting the particle method and the grid method calculations. Also, the numerical simulation of this welding process was conducted. As a result, a weld pool was swelled up by the transportation of molten metal droplets with the time evolution and it was solidified after the heat source passed. The plasma temperature distribution in the welding direction temporally became asymmetry at the start of the welding. This was because the metal vapor evaporated from a wire surface was transported to forward, preferentially in the welding direction. Furthermore, the iron vapor concentration on the weld bead became lower than forward side. Therefore, the arc temperature at forward of the wire became lower than backward because the radiation loss increased with increase in the iron vapor concentration. A few seconds after the start of the welding, the change of the arc plasma temperature distribution became smaller because a sufficient amount of a molten metal was supplied by the molten metal droplet transfer and the dent on the weld pool surface reduced.