Abstract: The microstructures of carbon/carbon composites (C/C) and refractory metal carbidedoped carbon/carbon composites (C/C+MC) were compared to elucidate their ablation mechanism in an arc jet heater. It was found that refractory metal carbides (MC) entered the crystallites of the composites to a depth of several microns from the ablated surface and obviously influenced the texture around them. The existence of MC aggravated not only the weight loss caused by mechanical peeling off but also the surface roughness of the samples. As a result, the ablation speed of C/C+MC was higher than that of C/C although MC decreased the total quantity of sublimed carbon.