This work investigated the effects of length and oxidation of multiwalled carbon nanotubes (CNTs) on theinterfacial strength, fracture toughness enhancement and conductivity for epoxy matrix composites by experiment. Firstly, aseven-step processing scheme was proposed to obtain highly dispersed CNT-epoxy composites. Subsequently, two feasiblemethods were presented to characterize the interfacial strength and used to study the effects of length and oxidation of CNTson the interfacial strength. Thirdly, based on the experimental results of the fracture toughness of CNT-epoxy composites, weproposed a new fracture theory for CNT-based composites which is contrary to the conventional fracture theory for fiberbasedcomposites. The experimental results show that the fracture toughness enhancement reaches maximum at the criticaloxidation time, when the interfacial strength equals CNT strength. At last, to obtain CNT-epoxy composites with both highmechanical and electrical properties, a feasible solution was put forward.