Non-equilibrium molecular dynamics (MD) simulations of isotopically impure carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) of varying lengths reveal that the relative decrease in thermal conductivity (k) in both the nanostructures are similar for constant isotope substitution. We find that the delocalized transverse modes in CNTs and the out-of-plane flexural modes in GNRs exert a substantial contribution to the thermal transport whereas the influence of isotope substitution on the phonon density of states (DOS) is similar in all directions. The vibrational spectra for different length scales consistently show that the energy modes shift to lower wave numbers reflecting the strong influence of mass disorder that reduces the energy transport through these nanomaterials, consequently lowering k. (C) 2014 Elsevier B.V. All rights reserved.