Carbon nanotube (CNT)-supported Pt metal nanoparticles were covered with silica layers by utilizing the successive hydrolysis of 3-aminopropyl-triethoxysilane and tetraethoxysilane on CNTs with Pt hydroxide. The CNT-supported Pt metal particles covered with silica layers (denoted as SiO2/Pt/CNT) were used as the electrocatalysts. SiO2/Pt/CNT electrocatalysts showed a high stability for the repeated potential cycling experiment, whereas Pt/CNT electrocatalysts were deactivated seriously for the experiment because of the growth of Pt metal particles in size. The silica layers in SiO2/Pt/CNT prevent the dissolution of Pt metal particles as well as the migration and agglomeration of Pt metal particles on the supports, which results in the improvement of the stability of Pt/CNT electrocatalysts.