Anisotropic mesophase carbon fiber(AMCFs) was exposed to isothermal oxidation in air and $CO_2$atmosphere, and burn-off rates have measured by TGA. The microstructure changes of oxidized carbon fibers, were observed by SEM. It was observed that oxidation rate in the air is over 100 times faster than that in $CO_2$atmosphere. The activation energy obtained in air was about 43.4 Kcal/mole in the temperature range of $600∼800^{circ}C$, and in $CO_2$was about 55.2 Kcal/mole in the temperature range of $950∼1200^{circ}C$. Therefore, the oxidation reaction in both atmospheres was under chemical reaction regime in the above temperature ranges. It was shown that the oxidation of the AMCFs is initiated at the end of fibers at high temperature($1100^{circ}C$) with developing the large pores, and the small pores are developed on the fiber surface at low temperature($900^{circ}C$). In conclusion, the oxidation of the AMCFs is progressed through the imperfection.