Effect of high temperature exposure (up to 1800 degrees C) on morphology of single- and multi-walled carbon nanotubes in Argon atmosphere has been studied using Raman spectroscopy and transmission electron microscopy. Although, as received nanotubes contained irregular graphene layers and other structural defects, microscopic observations revealed that heat treatment in Argon reduced the defect density and helped proper alignment of graphene layers. Raman spectra of as received and heat treated nanotubes strongly reinforced the microscopic observations. While, D-band to G-band intensity ratio in Raman spectra of 1800 degrees C heat treated multiwalled nanotubes reduced by similar to 43% over as received one, this ratio for heat treated singlewalled nanotubes was similar to 27% lower than that of the untreated specimen. Present study suggested that although, multiwalled nanotubes were structurally stable up to 1800 degrees C in an inert atmosphere having only a few nano-scale defects, singlewalled nanotubes suffered considerable damage at 1800 degrees C due to much thinner dimension than the former. (C) 2012 Elsevier Ltd. All rights reserved.