This study is on the electrocatalyst of MEA analysis technique to understand the degradation phenomenon in PEMFC. The ultra-microtome pretreatment method, FEG-SEM and TEM analysis methods were used for this purpose. Through these analysis methods, Pt coarsening and migration phenomenon is observed not only in electrode site but also in membrane site. This paper explain the catalyst degradation processes through the followings mechanisms ; (1) dissolution of Pt ; (2) diffusion of Pt ion ; (3) Pt ion chemical reduction in membrane ; (4) Coarsening of Pt particles (Ostwald ripening).