Hydroxyl-, amine-, and anhydride-terminated polyurethane (PU) prepolymer which were synthesized from polyether (PTMC) diol, 4,4’-diphenylmethane diisocyanate (MDI), and a coupling agent bisphenol-A, 4,4’-diaminodiphenyl sulfone (DDS), or benzophenonetetracarboxylic dianhydride (BTDA) were used to modify the toughness of bisphenol-A diglycidyl ether epoxy resin (DGEBA) cured with 4,4’-diaminodiphenyl sulfone. From the experimental results, it was shown that the modified resin displayed a significant improvement in fracture energy (G(IC)) and also in its interfacial shear strength with polyaramid fiber. It was more enhanced with increase of the PU modifier wt % content. The hydroxyl-terminated PU was found to be the most effective among those three prepolymers. In addition, the toughening mechanism was discussed based on the morphological and the dynamic mechanical behavior of the modified epoxy resin. Fractography of the specimen observed by transmission (TEM) and scanning electron microscopy (SEM) revealed that the modified resin had a two-phase structure. The existence of an unclean fiber surface after its fiber pullout test suggested that a ductile fracture might have occurred.