CH2OH-terminated regioregular poly(3-hexylthiophene) (P3HT) was grafted onto carboxylic groups of acid-oxidized carbon nanotubes (CNTs) via esterification reaction. The P3HT-attached CNTs (P3CNTs) are soluble in common organic solvents, facilitating an intimate mixing with free P3HT chains for strong electronic interactions. The optical and electrochemical properties of the resultant nanocomposite were found to be different from the conventional composite, in which the pristine CNT and P3HT were physically mixed together (P3HT/CNT). Electrochemical measurements on the onset oxidation and reduction potentials of the P3CNT showed positive shifts by 0.06 and 0.1 eV, respectively, with respect to the corresponding values of pure P3HT, indicating that P3CNT has a lower highest occupied molecular orbital (HOMO) energy level and a lower lowest unoccupied molecular orbital (LUMO) energy level than those of pure P3HT. Bilayer photovoltaic devices with a thin film of pure P3CNT as the electron-donor and C-60 as the electron-acceptor layer showed an increase in the power conversion efficiency by about 40% with respect to their counterpart based on pure P3HT.