Structural changes during oxidation of anisotopic mesophase carbon fiber(AMCFs) have been observed with TEM and XRD. It was shown that the AMCFs are three dimensionally stacked structure of turbostratic layers, and are non-homogeneous structure which consist of the highly ordered areas and the random areas. The $d_{ 002}$, Lc, and La of AMCFs which were measured by XRD were 3.44, 146, and $135AA$, respectively. It was observed that the oxidation initiated at the random areas, because the $d_{002}$ / decreased to the value of 3.41 $AA$ during initial oxidation stage. It was also observed that the La of the oxidized AMCFs increased up to 182 $AA$ during the whole oxidation process, and the $d_{002}$ of that increases up to 9.44 $AA$ when the burn-off is over the degree of 20%. Therefore, it was suggested that the micro-crystalline grew up by heat treatment effects during the fibers were oxidized, In addition, it was shown that there was difference in the measured value of La by XRD and TEM, in case of 39% oxidized fibers for example, the measured La was $ 165AA$ by XRD and in the range of 180∼220 $AA$ by TEM.