Influence of heat treatment of carbon fibers at temperatures of 2000 degreesC, 2400 degreesC, and 2600 degreesC on their microcrystalline structure and thermal stability in air atmosphere has been presented. Commercial carbon fibers, obtained from the petroleum pitch, were used In the research Microcrystalline structure of the fibers and their thermal stability were determined by Xray diffraction and by derivatographic methods, respectively. It has been proved that heat treatment increases the fibers resistance to the oxidation in gaseous and liquid phase, and that the oxidizing modification of the surfaces of non-thermally treated fibers substantially decreases their thermal stability.