The amounts of adsorbed oxygen on clean (0001) surface of natural graphite at 23°C has been determined by using the Knudsen flow capillary method. Graphite samples were the flattened (0001) single crystals (135 pieces, total weight: 0.146g, total geometrical surface area: 22.2cm² ) occur in South Africa and the scaly single crystals (36, 326 pieces, total weight 0.35g, total geometrical surface area 219cm²) occur in Guatemala. The results obtained indicate that the saturated quantities of adsorbates are 1.7×10¹⁵ (molecules/cm²) for the South Africa-graphite and 9×10¹⁴ (molecules/cm²) for the Guatemala-graphite, respectively. These data suggest that oxygen molecules are adsorbed on the basal plane of graphite surface.