The Raman spectra for single-walled carbon nanotubes (SWNTs) at different temperatures are studied. We find that the G peak position shifts to low frequency with increasing temperature. The variation rate of the peak frequency as a function of temperature is a factor of 2–3 larger than the corresponding values for multiwalled carbon nanotubes and highly ordered pyrolytic graphite from room temperature to 673 K. The line shapes of the radial breathing mode features are also found to be sensitive to temperature. Softening of the interatomic force constant due to thermal expansion of C–C bonds and relaxation of the weak van der Waals interaction between the SWNTs in a bundle with increasing temperature are suggested to be the main origins of the reversible spectral variations.