Post-thermal treatment prior to carbonization was used to modify polyacrylonitrile stabilized fibers, and in this study, the effects of this modification on the structures of the stabilized fibers and the resulting carbon fibers were investigated. The stabilized fibers were modified by thermal treatment under a nitrogen atmosphere at different temperatures, followed by continuous carbonization to obtain carbon fibers. The reaction mechanism and structural changes in the stabilized fibers during the post-treatment process were investigated by C-13 solid-state nuclear magnetic resonance and X-ray diffraction analysis. The degree of stabilization of the post-treated stabilized fibers is higher than that of the as-received stabilized fibers and increases with increasing post-treatment temperature. The crystal width of the resulting carbon fibers increases continuously, whereas the crystal thickness remains nearly the same with increasing post-treatment temperature. When the post-thermal treatment for modification is conducted at 300 A degrees C, the stabilized fibers possess the highest degree of orientation. This property is inherited by the resulting carbon fibers, which exhibit the highest degree of orientation and optimal mechanical properties.