This paper presents an effective numerical model capable of simulating the filling stage of the gas-assisted injection molding process, particularly the gas penetration phenomenon involving the gas-polymer interaction. A Galerkin Finite Element model is used to model the polymer flow during the filling and gas penetration stages. The polymer flow front and the gas-polymer interface are tracked using a volume tracking technique. Finally, a finite difference technique has been used to calculate the temperature across the thickness. Comparison between numerical simulation predictions and the experimental data for a plate mould as well as for a complex part is presented.