A full 3D finite element method has been used to understand how nano-clay particles affect the mechanical properties of an epoxy/clay nanocomposite. The epoxy/clay nanocomposite has been modelled as a representative volume element (RVE) containing intercalated clay platelets that internally delaminates at the gallery layer upon satisfying an energy criterion, and an epoxy matrix that is elastic-plastic. A cohesive traction-displacement law is used to model the clay gallery behaviour until failure. For clay volume fractions >1%, clay particle interaction is observed to develop during uniaxial tension, the nanocomposite stiffness becomes non-linearly dependent on the clay volume fraction, and the Mori-Tanaka model overestimates the stiffness. Failure of the clay gallery is not observed and is believed to have no influence on the ultimate tensile strength of the nanocomposite.