Gas-assisted injection molding can effectively produce parts free of sink marks in thick sections and free of warpage in long plates. This article concerns the numerical simulation of melt flow and gas penetration during the filling stage in gas-assisted injection molding. By taking the influence of gas penetration on the melt flow as boundary conditions of the melt-filling region, a hybrid finite-element/finite-difference method similar to conventional-injection molding simulation was used in the gas-assisted injection molding-filling simulation. For gas penetration within the gas channel, an analytical formulation of the gas-penetration thickness ratio was deduced based on the matching asymptotic expansion method. Finally, an experiment was employed to verify this proposed simulation scheme and gas-penetration model, by comparing the results of the experiment with the simulation. (C) 2003 Wiley Periodicals, Inc.