Pt catalysts supported on Vulcan XC-72 carbon have been prepared with H₂PtCl₆ㆍ6H₂O as metal precursor. Measurements of surface area and crystallite siize by electrochemical method and by X-ray diffrac-tion showed that the supported platinum crystallites grew when in contact with a electroconducting solution. This study shows that the platinum area loss depends on such factors as an initial crystallite size, platinum concentration, temperature and potential of electrode. A soluble platinum in conducting solution was redepos-ited on the electrode surface at cathodic potential, but there was no detectable area losses due to heating in nitrogen, argon or hydrogen at 400℃. In this study we will present the agglomeration phenomena of platinum particles and the reason of growth of platinum particles.