This paper reports a study of the surface composition of carbon fibers treated by various methods by means of X-ray photoelectron spectroscopy (XPS). C//1//SX-ray photoelectron spectra showed that after the surface treatment of carbon fibers, the carbon atoms in the hydrocarbon were changed into oxygen-containing groups, that is, the results of surface oxidation and concentration increased with time but finally reached a constant level. Comparing experimental results for the treatments used, we found that all of these methods resulted in concentrations of oxygen groups on the surface.