We have developed a novel preparation procedure for an electrocatalyst layer with high utilization of catalyst for polymer electrolyte fuel cells. A commercial Pt catalyst supported on high surface area carbon black (Pt/CB) and Nafion ionomer solution was heated in an autoclave at 200 C, followed by quenching to form the ink of the mixture. It was found that the cathode prepared with the new catalyst ink exhibited very high performance, i.e., high catalyst utilization and improved gas diffusivity. The microstructure analysis indicated that the autoclave treatment promoted an effective introduction of Nafion ionomer into primary pores of Pt/CB agglomerates, in which ca. 90% of Pt catalysts were supported. It was clearly observed by scanning transmission electron microscopy that Nafion ionomer was distributed more uniformly inside Pt/CB agglomerates, compared with those simply mixed with a ball mill in a conventional manner.