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The influence of the type of epoxy resins (E-51, F-51) on the mechanical properties and microstructure of carboxyl-terminated butadiene nitrile rubber (CTBN) toughened epoxy resins was investigated by determination of adhesive strength and toughness as well as observation of scanning electron microstructure (SEM). The results indicate that the adhesive strength of CTBN toughened E-51 system is superior to CTBN toughened F-51 system. However, CTBN toughened E-51 system is inferior to CTBN toughened F-51 system in the toughness. This difference is related to the microstructure of two toughened systems. For CTBN toughened F-51, larger cavities and rubber particles are found in the system. The larger cavities easily cause stress concentration and result in decrease of the adhesive strength. While the larger rubber particles may better terminate the development of crazing and shear banding and result in increase of the toughness.