In the present study of resin transfer molding, an effective gap located between the side wall and the fiber mats is proposed. The concept of the effective gap leads to a permeable boundary condition on the interface between the fiber mats and the side wall. The porosity is no longer assumed to be a constant : in the whole domain. Results show that the effective gap concept can explain the phenomena of edge effects reasonably. In our model, the filling process is simulated by the body-fitted finite element method and the limitation of the time increment for the quasi-steady state process is discussed in detail.