A model was developed to simulate the Reaction Injection Molding Process of unsaturated polyester rosin. The model simultaneously solved the balance equations for chemical species, momentum and energy by a computer simulation. Unsaturated polyester with compositions of phthalic anhydride, propylene glycol and fumaric acid with styrene was used. The reaction of unsaturated polyester resin was initiated by methyl ethyl ketone peroxide and promoted by cobalt naphthenate. The reaction kinetics was studied in a quasi-adiabatic reactor, and an empirical viscosity function was also developed. The effects of initiator concentration, fill time, mold thickness and overall heat transfer coefficient were evaluated through computer simulation. Actual Reaction Injection Molding experiment using acrylic mold was carried out and the experimentally obtained temperature profiles were compared with computer simulation results.