Platinum particles with mean particle size from 0.8 to 1.8nm were loaded on carbon black sup-ports by either impregnation or colloidal method. In order to change particle size and intercrystallite distance of platinum, the surface area of carbon was varied and the prepared Pt/carbon catalysts were treated at 800℃ in helium atmosphere. An half-cell test was conducted with Teflon-bonded Pt/C electrodes in order to investigate the oxygen reduction current density in 100% H₃PO₄ at 190℃. Catalytic activity was strongly dependent on the platinum particle size; the maximum in mass activity was observed at about 3nm and the specific activity increased with increase in particle size. The effect of intercrystallite distance on the activity was mainly contributed from the effect of platinum particle size. It was possible to obtain the oxygen reduction current density of 270mA per cm² electrode at cathode potential of 0.7V/RHE.