The temperature dependence of the morphology of single wall carbon nanotubes (SWNTs) has been studied by Raman spectroscopy at different temperatures by changing the incident laser power. It is shown that a high power laser irradiation treatment of as-grown samples anneals the SWNTs, improving their structural order and perhaps also removing adsorbed gases. As a result, a significant increase in the Raman cross-section of the nanotubes upon laser irradiation can be observed, for both metallic or semiconducting tubes. The investigation of the power level dependence of the Raman spectra also reveals that smaller diameter nanotubes are burned off first, increasing the mean diameter of the nanotubes in the sample. (C) 2002 Elsevier Science B.V. All rights reserved.