The support effect of carbon nanotubes (CNTs) on the performance of CO tolerant electrocatalysts for PEFC was studied using CNTs with and without defect preparation, carbon black, and fishbone-type CNTs. Pt-Ru/defect-free CNTs revealed very high CO tolerance under 100 ppm CO in the half-cell system of hydrogen oxidation. The catalytic activity was maintained under 100 ppm level CO with good reproducibility. On the other hand, the hydrogen oxidation current on Pt-Ru/defective CNTs, Pt-Ru/fishbone-type CNTs and Pt-Ru/VulcanXC-72C decreased largely with increasing concentration of CO up to 100 ppm. It is thus considered that the carbon substrates significantly affect the CO tolerance of anode electrocatalysts in PEFC. This is ascribed to the flat interface between CNTs and metal catalysts, at which the electron transfer occurs, and this interface would modify the catalytic properties of Pt-Ru particles.