Carbon nanotube-supported Pd catalyst (Pd/CNT) was covered with silica layers a few nanometers thick by successive hydrolysis of 3- aminopropyltriethoxysilane (APTES) and tetraethoxysilane (TEOS) to improve the durability of the Pd catalyst under cathode conditions of polymer electrolyte fuel cells (PEFCs). The coverage with silica layers resulted in the improvement of durability of Pd/CNT during the potential cycling between 0.05 and 1.20 V (vs. RHE) in aqueous HClO 4 electrolyte. The catalytic activity and durability of silica-coated Pd/CNT strongly depended on silica layer thickness in the catalysts. The coverage with thicker silica layers improved the durability of Pd/CNT but the catalytic activity of the catalysts for the oxygen reduction reaction became lower with silica layer thickness. The coverage of Pd/CNT with silica layers of ca. 3 nm thickness is preferable from a viewpoint of the catalytic activity and durability.