Phenolic hydroxyl-terminated (HTPU) and aromatic amine-terminated (ATPU) PU modifiers were prepared by reacting two different macroglycols (PTMG, polytetramethylene glycol, M(n) = 2000, and PEA, polybutylene adipate, M(n) = 2000) with 4,4’-diphenylmethane diisocyanate(MDI), then further coupling with two different coupling agents, bisphenol A or 4,4’-diaminodiphenyl sulfone (DDS). These four types of PU prepolymers were used to modify the epoxy resin with 4,4’-diaminodiphenyl sulfone as a curing agent; From the experimental results, it was shown that the values of fracture energy, G(IC), for PU-modified epoxy were dependent on the macroglycols and the coupling agents. Scanning electron microscopy (SEM) revealed that the ether type (PTMG) of PU-modified epoxy showed the presence of an aggregated separated phase, which varied between 0.5 mu m and 4 mu m in the ATPU (PTMG) and between 1 mu m and 1.5 mu m in HTPU (PTMG) modified system. On the contrary, the ester type (PEA) PU-modified epoxy resin showed a homogeneous morphology and consequently a much smaller effect on toughening for its good compatibility with the epoxy network. In addition, it was found that the hydroxyl-terminated bisphenol A as a coupling agent improved fracture toughness more than the amine-terminated DDS because of effective molecular weight buildup by a chain extension reaction. The glass transition temperature (T-g) of modified epoxy resin as measured by dynamic mechanical analysis (DMA) was lower in PTMG-based PU than in a PEA-based PU series with the same weight of modifier.