In this study, the dissolution mechanism of Pt and loss of Electrochemical Surface Area (ECSA) at cathode condition of PEMFC have been clarified by using ICP-Mass and amount of charge of hydrogen under potential deposition (H upd) respectively. ICP results showed that Pt dissolution was accelerated at potential above 1.0V vs.SHE when potential was cycled at 10 and 100 mVs-1. Amount of dissolved Pt increased as the sweep rate decreased. P1 dissolved at 10 mVs-1 was about 2 - 3 times more than that at 100 mVs-1 in the upper potential limit of 1.2 and 1.4V. This increase of the dissolution can be explained by increase of oxidation time and decrease of the amount of redeposition. On the other hand, Hupd analysis revealed ECSA of Pt didn't change in all potential range at 100 and 1000 mV s-1, but drastically decreased at 10 mVs-1. Particularly, ECSA decreased at rate of approximately 40% as compared to that before 100cycles of CV in the potential range 0.0 ∼1.0 or 1.2V at 10 mVs-1. AFM observation showed that particle grain growth was one of important factor of this surface area loss.