The hydrogenolysis up to 90% carbon conversion of graphitized carbon black on platinumcatalyst in a flow reactor, at 873-1073K under 101 Kpa hydrogen pressure, has shown that the reaction rates（g carbon reacted (initial surface area）^-1(time)^-1) reached the maximum in the half way of 90% conversion. The change of Pt particle size over this range of the conversion was also measured by H₂ chemisorption, X-ray line broadening and TEM. Pt particle size calculated from H₂ chemisorption was too large due to the decrease of H₂ uptake on the Pt surface. In the early stage of conversion, Pt particle sizes measured by XRD and TEM did not change significantly. The mechanism of Pt catalyzed carbon gasification is discussed in the light of these observation.