Engineering analysis of reaction injection molding (RIM) process of polyurethane was carried out by simultaneously solving the balance equations of mass, momentum, and energy by a computer simulation. The polyurethane system evaluated was the reaction system of 4,4''-diphenylmethane diisocyanate(MDI) and poly(propylene glycol) (M_n:752) catalyzed by dibutyltin dilaurate (T-12). Basic data for simulation such as the reaction rate constants and the viscosity variations on time were obtained empirically by running adiabatic reaction. Temperature and conversion profiles at different points of the mold under constant temperature of heat transfer medium were obtained through computer simulation. The effects of reaction rate constant at feed temperature on conversion and temperature at fill time were discussed.