In this study, a multiscale model fully representing mechanical deformation is developed for the identification of geometrical and mechanical properties of the interfacial layer in single-walled carbon nanotube (SWNT)-epoxy nanocomposite. The mechanical properties of nanocomposite reinforced with SWNTs are derived using all-atom molecular dynamics (MD) simulations for different diameter nanotubes under constant composition conditions. A nanotube size effect on axial stiffness along the nanotube alignment direction is clearly observed, whereas the transverse axial and shear stiffness components are less than the corresponding values of neat polymer. Through the analysis of deformation energy and its distribution inside the nanocomposite unit cell, the presence of an inner soft and slippery polymer layer at the vicinity of the nanocarbon surface is revealed. Taking account of this unusual reinforcing effect using a finite element (FE) model, we implicitly solve for the size and mechanical properties of the effective interphase, which has an equivalent deformation energy around the nanotube, as well as the global elastic stiffness of the nanocomposite that is equivalent to the corresponding value from MD simulations. The equivalent continuum model thus properly predicts the local stress distribution at the adsorbed polymer-SWNT interface as well as the overall mechanical properties of nanocomposite, along with their inherent nanotube size effect. (C) 2016 Published by Elsevier Ltd.