Multiwalled carbon nanotubes (CNTs) are used as reinforcement in epoxy modified with adduct, that is, block copolymer of carboxyl-terminated butadiene acrylonitrile (CTBN) and diglycidyl ether of bisphenol A (DGEBA). Microstructure analysis reveals uniform dispersion of a phase with spherical morphology in a continuous phase. The mean diameter of the spherical particles increased with CNT and adduct content. At 30 wt % adduct, adding CNTs results in exceptional growth of spherical particles. Loss factors in the first two modes were determined by using free vibration test in cantilever mode. Significant increase in loss factor is obtained for the epoxy nanocomposites modified with 30 wt % adduct and at all wt % CNTs (i.e., 0.5, 1, and 2.5). The vibration damping mechanism in this nanocomposite is likely due to the relative sliding of spherical particles during dynamic loads, which dissipates energy. The reduction in the distance between spherical particles as well as the reduction in the elastic modulus of continuous phase facilitate relative sliding. © 2019 American Chemical Society.