Carbon-fiber-reinforced carbon matrix composites (C/C) are one of the promising candidates for heat-resistant materials because they have high strength and good creep resistance at high temperatures. Their application as a material for the turbine blades of a proposed chemical gas turbine system would increase the thermal efficiency. In this study, the time changes in weight loss of C/C composites have been measured in the combustion field at several equivalence ratios. It was found that the degradation of C/C composites was much less at a high equivalence ratio. Even in a fuel-rich combustion field, however, C/C composites cannot perfectly be free from the degradation. To enable this material to be used at higher temperatures, C/C composites were coated with SiC. The effects of coating layers on oxidation resistance have been investigated experimentally. The surfaces of some specimens before and after the exposure in a combustion gas stream of methane-air were observed by SEM and were also evaluated by Raman spectroscopy. The layer coated by the CVD method was easily peeled off from a substrate. The layer of specimens coated by the pack cementation method has been found to be stable. However, the substrate was degraded because of oxygen penetration through the pores in the coating layer. In order to cover the pores, the specimens were additionally coated with glass materials. This double-coating technique seems to be promising to improve the oxidation resistance of C/C composites in the combustion field.