The multi-walls carbon nanotubes reinforced epoxy resin was treated with ultrasonic irradiation. The effects of ultrasonic irradiation on the viscosity, mechanical properties and fracture morphologies of composites were investigated. The results show that at the same temperatures, the viscosity of multi-walls carbon nanotubes reinforced epoxy resin gradually decreases with the increasing of ultrasonic irradiation time; When the ultrasonic irradiation time is 5 min, the tensile strength, flexural strength and tensile shear strength of epoxy resin reach the highest value, which were increased respectively by 37%,167% and 86%; Ultrasonic irradiation can make the tensile fracture morphology show more ductile fracture characteristics.