The curing process of hexamethylene diisocyanate-based polyurethane has been monitored by applying FTIR and DSC methods. A general relationship between glass-transition temperature (T-g) and conversion of curing process has been obtained. This suggests that the reaction path and the relative reaction rates are independent of the curing temperature. The reaction kinetics of the system is analyzed using the T-g data converted to the conversion of the curing process. A set of experimental data and one theoretical model of T-g versus chemical conversion are presented to prove the assumption where a direct one-to-one relationship between the T-g (as measured) and the chemical conversion is obtained. Apparent activation energies (E-alpha) obtained by applying three different methods suggest good agreement.