The modification of bisphenol-A-based epoxy resin with isocyanates was investigated. To increase the crosslinking density by a urethane reaction and to conduct the normal cross-linking reaction between the epoxide and the amine during the cure cycle, blocking the isocyanates with active hydrogens is necessary, which may be deblocked and reacted at elevated temperature. The modified reactions involving diisocyanates and hydroxyl groups generated from epoxy cured with an amine system were studied. This mechanism can be identified by the variations of infrared spectra in the carbonyl group stretching region. Furthermore, the effect of the blocked isocyanate incorporation with amine on the reactivity of epoxy was also studied. The thermal and the mechanical properties were characterized. It was found that the glass transition temperature increased with an aromatic blocked-diisocyanate content. The mechanical properties were improved by blocked diisocyanate. The morphology was also investigated, which showed that a homogeneous structure existed in the modified system.