With the aim of developing durable electrocatalysts for PEFC, their durability with different carbon supports was evaluated electrochemically. During our study, Pt deposited Ketjen Black carbon was found to lose most of electrochemical active surface area after the potential cycle tests, and then a possible mechanism for that was investigated. We believe two different mechanisms probably occur at the same time. Many of Pt particles are dissolved into the electrolyte solution either as Pt ion or Pt nanoparticles. Some of Pt particles are also embedded into the hole formed on the carbon surface owing to carbon corrosion. However, the effect of these two factors was rather small for Vulcan carbon. Moreover, on the graphitized surface, such problems were mostly overcome. Consequently, graphitization of the carbon surface is a simple but useful technique to improve the durability of electrocatalysts.