Hydroxyl-, amine-, and anhydride-terminated polyurethane (PU) prepolymers, which were synthesized from polyether [poly(tetramethylene glycol)] diol, 4,4 ' -diphenylmethane diisocyanate, and a coupling agent, bisphenol-A (Bis-A), 4,4 ' -diaminodiphenyl sulphone (DDS), or benzophenonetetracarboxylic dianhydride, were used to modify the toughness of Bis-A diglycidyl ether epoxy resin cured with DDS. Besides the crystalline polymers, poly(butylene terephthalate) (PBT) and poly(hexamethylene adipamide) (nylon 6,6), with particle sizes under 40 mum were employed to further enhance the toughness of PU-modified epoxy at a low particle content. As shown by the experimental results, the modified resin displayed a significant improvement in fracture energy and also its interfacial shear strength with polyaramid fiber. The hydroxyl-terminated PU was the most effective among the three prepolymers. The toughening mechanism is discussed based on the morphological and the dynamic mechanical behavior of the modified epoxy resin. Fractography of the specimen observed by the scanning electron microscopy revealed that the modified resin had a two-phase structure. The fracture properties of PBT-particle-filled epoxy were better than those of nylon 6,6-particle-filled epoxy. Nevertheless, the toughening effect of these crystalline polymer particles was much less efficient than that of PU modification.