An investigation is performed to study the influence of boron treatment on carbon fiber : its oxidation behaviors, kinetics, and structure and properties at a high-temperature atmosphere in air. PAN-based carbon fiber is treated with either liquid organoborate or inorganic berate, which elevates the decomposing point above 260 degrees C and doubles the average oxidation activation energy from about 100 kJ/mol of the untreated fiber to above 200 kJ/mol of the treated fiber, with the first order of the fiber oxidation reaction under nonisothermal conditions. The boron coating formed on the fiber surface after the treatment may cap off the specific surface active sites to function as a diffusion barrier that inhibits the oxidation, and its main structure is identified as boron oxide by means of X-ray diffraction and XPS analyses. During the process, the mechanical properties develop slightly; however, the morphology structure observed by SEM changes greatly.