The residue compounds of sulfuric acid-carbon fiber system were obtained from P100, M40, TZ307, and T300 fibers. The fibers which easily formed intercalation compound had small change of d002 value, but, large change of diameter of fiber. It was considered that the cracks in carbon fiber were due to the this conflicting result. The resistivities of residue compound of carbon fibers were increased with the large change of diameter of fibers. It is thought that the reason of our results is increasing defects such as crack in fibers. From the UV reflection spectra, it was found that reflectances of residue compounds were all decreased and the residue compounds were more stable than intercalation compounds.