The redox reaction of cytochrome c was not observed at as-grown, high-quality, single-walled carbon nanotubes (SWCNTs) synthesized onto a gold electrode surface. On the other hand, at a gold electrode modified with SWCNTs with oxidized carbon species such as C-O, C=O, and O-C=O induced by UV-ozone treatment, a well-defined diffusion-controlled redox wave of cytochrome c was observed. The results clearly indicated that oxidized carbon species such as C-O, C=O, and O-C=O play an important role for the fast heterogeneous direct electron-transfer reaction of cytochrome c with CNTs.