The temperature distributions in current-carrying carbon nanotubes have been measured with a scanning thermal microscope. The obtained temperature profiles reveal diffusive and dissipative electron transport in multiwalled nanotubes and in single-walled nanotubes when the voltage bias was higher than the 0.1-0.2 eV optical phonon energy. Over 90% of the Joule heat in a multiwalled nanotube was found to be conducted along the nanotube to the two metal contacts. In comparison, about 80% of the Joule heat was transferred directly across the nanotube-substrate interface for single-walled nanotubes. The average temperature rise in the nanotubes is determined to be in the range of 5-42 K per microwatt Joule heat dissipation in the nanotubes. © 2009 American Institute of Physics.