The study deals with ultrasonic treatment of liquid epoxy oligomer filled with multiwall carbon nanotubes of various diameters. We studied the effect of treatment time on fracture toughness and failure mode of cured epoxy compositions. Ultrasonic treatment time varied from 1 to 4 hours. Carbon nanotubes concentration was 0,5% for all cases. It was shown that ultrasonic treatment affected the properties of nanomodified composite: maximum values of fracture toughness for epoxy binders with various carbon nanotubes types were obtained for treatment times different from each other. Besides, a solvent was selected which was the most suitable for introducing the nanotubes into epoxy resins.