Abstract: Toughening effect of polyether polyol on epoxy resin was analyzed by means of mechanical, thermal, viscosity testing and scanning of electrical microscope. Results show that polyether polyols are phaseseparated out during the curing process of epoxy resin. Polyether polyol with relatively low molecular weight shows a better toughening effect than those with high molecular weight. Polyether polyol with end block of PEO, which is compatible with epoxy matrix, also shows remarkable toughening effect because of a good interfacial compatibility.