A feasibility study has been conducted to determine whether mathematical models can be used for the numerical simulation of MAG (Metal Active Gas) arc welding. In the present work, a three-dimensional (3-D), non-stationary thermal model for butt joint is developed. The transient temperature distribution in the base metal is numerically analyzed to estimate the molten pool size using a finite differential model based on the heat flow equation, and the theoretical configuration of the molten pool is calculated, taking account of the balance of gravity, surface tension and arc pressure. To evaluate the validity of the model analysis, the calculated results are compared with experimental results for MAG welding. Good correspondence is demonstrated between experiment and calculation. According to the software, the weld profile and temperature history in the work-piece for a set of process parameters is easily estimated. To predict the MAG arc welding phenomenon comprehensively, the developed welding process model has been linked to a deformation model. Accordingly, there is the possibility that the software is useful as an engineering tool in a manufacturing environment.