A control volume technique based on the finite difference method is used to characterize the flow behavior in resin transfer molding (RTM) of composite structures. Resin flow through fiber mats is modeled as a two-phase flow through porous media. The transient rime terms are considered, and the concept of the fraction volume of fluid (VOF) is applied in order to accurately describe the behavior of the free boundary at the interface between the two fluids involved in the process, the resin and the air. Flow experiments in a transparent mold were performed to verify the numerical model. Experimental results on flow behavior of EPON 826 epoxy resin into irregular mold cavity with fiberglass mats agree well with the present numerical simulation. Several parametric studies using the developed model are conducted to investigate the effects of injection pressure and mold design on resin flow pattern, mold filling time, and pressure distribution inside the mold.