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 fillet welding is developed. The transient temperature distribution in the base metal is numerically analysed to estimate the molten pool size using a finite difference 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. The developed model can be applied to various welding processes such as T-joint welding and fillet welding with torch weaving. 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. A computer software for MAG welding process model is introduced briefly in the last part of the paper. According to the software, the weld profile and temperature history in the work-piece for a set of process parameters is easily estimated, and the molten pool dynamics are reproduced visually using an animation technique.