Abstract: Toughening of an epoxy resin system for glass and carbon fiber composites is investigated.The system consists of butadiene-acrylonitrile rubber with random carboxyl groups (CTBNX),epoxy resin of diglycibyl ether of bisphenol A (DGEBA) and 2-ethyl-4-mnthy limidazol.The authors' previous studies1,2,3 showed the following points:(1) A toughened epoxy resin can be prepared with CTBNX or GTBNX-bisphenol A to form two-phase system.The toughened epoxy resin exhibits significant improvements over the un modified ones by 2 to 10 times in impact strength,by 0.5 to 11.5 times in modulus of toughness(area under the stress-strain curve under tension) and by 1 to 4 times in fracture surface energy.(2) When the toughened epoxy resin is used as matrix for glass fiber composites,it improve longitudinal and transverse impact strength, fracture surface energy, candidate stress intensity factor K_Q in unidirectional laminates and other resin-dependent mechanical properties.