Structural reaction injection molding (SRIM) was used to produce polyurethane composites containing random continuous glass fiber mats. A long rectangular mold was used to carry out the SRIM experiments. 4,4′-diphenylmethane diisocyanate and poly(propylene oxide) triol were used to formulate a thermoset polyurethane system. Dibutyltin dilaurate was used as a catalyst. A second order Arrhenius equation described the PU polymerization kinetic data obtained from the adiabatic temperature rise measurement. A viscosity as a function of temperature and conversion was developed using rheometer data. The pressure rise at the gate was measured during filling. The flow behavior within the mold was described by Darcy's law and the Kozeny's equation. The temperature profile within the mold measured by thermocouples during filling and curing coincided fairly well with the simulation results. The thermal transient problem at the wall was solved using the overall heat transfer coefficient, and it was analyzed as a function of Biot number. The dimensional stability of the fiber reinforced PU parts was excellent compared to the pure PU parts.