This work provides information about polymer composite manufacturing by using liquid composite material molding, with particular reference to resin transfer molding process (RTM). Herein, several topics related to porous media, fluid flow, mathematical modeling, computational methods, composite manufacturing and industrial applications were presented. Simulation of resin flow into a fibrous (reinforcement) inserted in a parallelepiped mold has been performed, using the Ansys FLUENT®software, and different results of resin volumetric fraction, stream lines and pressure distribution inside the mold, and volumetric fraction always flow rate (inlet and outlet gates) of the resin, as a function of filling time, have been presented and discussed.