The electric field influence of 1 volt on the typical polymer electrolyte fuel cell catalyst system carrying Pt clusters on amorphous carbon particles have been elucidated by transmission electron microscope observation. The same position was observed before and after exposing to the electric field. Surface of Pt clusters were covered with a few graphitic layers. The slight graphitization of amorphous carbon particles has been also occurred. The growth of Pt clusters has been discussed as the aggregation of the Pt atoms which exist on carbon particles.