A carbon fiber tow subjected to internal resistance heating often breaks when its surface temperature is raised higher than about 2200 degrees C. In this study, the cause of the tow breakage was investigated. Internal resistance heating gives rise to a large temperature distribution in the tow cross section, the temperature being at a maximum in the central region of the tow cross section. Because of this temperature distribution, carbon filaments in the tow are sublimated from the central region of the tow cross section, leaving a growing cavity; as a result the tow breaks. Some of the sublimated carbon deposits on the filaments in the low temperature region surrounding the sublimating region, forming a laminar texture around the filament surface. The structure of the deposited carbon was investigated by reflected light microscopy, by scanning electron and transmission electron microscopy, and by electron and wide-angle X-ray diffraction. The deposited carbon was found to be composed of carbon layer stacks with low in-plane stacking regularity.