The plasma arc welding can realize high speed welding, deeper weld penetration and, furthermore, samller thermal distortion. For these reasons the plasma arc welding is employed for keyhole welding. However, it has been pointed out that it is difficult to obtain the stability of the welding. In this study, we have developed an unified plasma arc welding model for analyzing the welding mechanism and influence of the torch design and operation conditions of the arc on the welding process. In this paper, the keyhole welding of a thick aluminum plate employing the plasma arc was numerically analyzed for clarifying the mechanism determining the keyhole size which is important for improving the stability of the process. As a result, it was found that keyhole size is determined mainly by a force balance between the surface tension of a weld pool and metal vapor pressure.