Abstract: The oxidation behaviors of the homemade high thermal conductive mesophase-pitch-based carbon fiber (CF_MP) at different time and temperature were investigated using the polyacrylonitrile-based carbon fiber (M55J) as the control group. The results show that CF_MP exhibits a fold radiation structure in the outer part and onion skin structure in the inner part. The CF_MP has a well-developed graphite crystallite and a high degree of orientation. Oxygen atoms preferentially diffuse in the microcracks and micropores in the fold radiation carbon textures of CF_MP and react with them resulting in radial cracks and localized pits. In the low temperature oxidation stage, the oxidation behaviors of the fibers are controlled by the carbon-oxygen chemical reaction. Because the active site concentration in graphite crystallite of CF_MP is lower, its initial reaction temperature is higher than that of M55J, and its oxidation mass loss rate is relative lower. In the high temperature oxidation stage, the oxidation behaviors of the fibers are controlled by oxygen diffusion. The oxidation mass loss rate of CF_MP is higher than that of M55J because there are more oxygen diffusion paths in the CF_MP. Moreover, because there are more and larger microstructural defects in CF_MP after oxidation, the strength retention rate of the CF_MP is only 78%, which is lower than that of M55J (85%). This study provides certain technical and theoretical references for the structural design and actual service of high thermal conductive C/C composites.