Abstract: How to toughen 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 diglycidyl ether of biphenol A（DGEBA）and 2-ethyl-4-methylimidazol.The main objectives of this research are:（1）to toughen epoxy resins with commercially available elastomer CTBNX;and（2）to determine the effectiveness of this CTBNX-modified epoxy resin system in fiber reinforced composites.The authors'previous studies（4,5）show that the toughened epoxy resin can be prepared with CTBNX or CTBNX-bisphenol A to form a two-phase system.The toughened epoxy resins exhibit significant improvements over the unmodified ones in impact strength,modulus of toughness（area under the stress- strain curve）and fracture surface energy,by as much as 2 to 10 times,0.5 to 11.5 times and 1 to 4 times respectively.This paper presents the research results of the effects of matrix toughness on the crack resistance,impact strength and mechanical properties of unidirectional glass fiber composites and describes the technique of measuring the fracture toughness KQ of fiber composites.Test results show that:1.In the unidirectional glass epoxy composites,the increases of transverse and longitudinal impact strengths,fracture surface energy and candidate stress intensity factor KQ depend mainly on enhancement of the resin matrix toughness,but are not so large as in bulk resins and only modest,though still the increases obtained are significant.Wtih enhancing the matrix toughness,short beam interlaminar shear strength and all transverse mechanical properties except tensile modulus of composites increase,while all longitudinal mechanical properties and flexural properties change inappreciably.2.The use of double edge notched tension specimen（DEN）for determining fracture toughness of unidirectional fiber composites is recommended.