Abstract: MWCNTs grown by chemical vapor deposition on SiO₂/Si substrates were investigated by Raman spectroscopy, transmission electron microscopy (TEM), Auger spectroscopy, and X-ray photoelectron spectroscopy before and after an annealing at 390℃ for 120 min in air or chemical treatment with a HCl solution. The Raman spectroscopy was focused on the low-frequency (250-300 cm^-1) band. It is found that the positions and full widths at half maximum of the peaks forming the 250-300 cm^-1 Raman band change little with the annealing or chemical treatment. The measured inner diameters of small-diameter CNTs from TEM agree well with those from Raman spectroscopy. These indicate that the low-frequency band originates from the radial breathing oscillations of carbon atoms in the inner walls of small-diameter MWCNTs.