Prepolymers were prepared by the reaction of 3,9-dihydroxyethyl-3’,9’-dibenzyl-1,5,7,11- tetraoxaspiro(5,5)undecane with 4,4’-diphenylmethane diisocyanate (MDI) and 1,6-hexamethylene diisocyanate (HDI). The number-average molecular weights of the prepolymers can be controlled by changing the mole ratios of spiro compound and diisocyanates. Kinetic studies of the cure reaction for the epoxy resin system modified with or without prepolymers were followed by a HLX-1 dynamic torsional vibration apparatus. The results indicated that gel time (t(g)) and activation energy (E(a)) increased as the content of prepolymers in the epoxy resin system increased. A difference with the cure reaction of the pure epoxy resin, the second-order reaction for the epoxy resin modified with the prepolymers, was obtained. Rate constants (it) of the cure reaction are 0.231 min-l for the epoxy resin, and 0.312 min-l for the modified epoxy resin. The mechanism of the cure reaction was discussed.