The surfaces of wool fibers are modified with N-doped TiO2 nanoparticles by treating the fibers with tetrabutyl titanate and ammonium chloride under low temperature hydrothermal conditions to obtain wool fibers with photocatalytic functions in the visible light spectrum. The effects of nitrogen and sulfur in amino acids in keratin on the photocatalytic activity of TiO2 particle coated wool fibers are investigated. Changes of various fiber properties such as tensile strength, photocatalytic activity, and self-cleaning performance of untreated, TiO2 coated and N-doped TiO2 coated wool fibers are studied. It is found that N-doped anatase TiO2 nanoparticles with an average grain size of 11.2 nm are synthesized and simultaneously grafted onto the wool fibers. After treatments, the crystallization index of the wool fibers is slightly reduced. The capability to protect against ultraviolet radiation is much enhanced. The performances of photocatalytic degradation of methylene blue dye and self-cleaning of red wine under both UV and visible light irradiation are endowed. It is also found that wool fibers coated with TiO2 particles without being doped by nitrogen still have apparent photocatalytic reactions and self-cleaning effects under visible light irradiation due to the formation of C-Ti3+, O-Ti3+, and N-Ti3+ bonds between TiO2 and wool keratin on the wool fiber surfaces. Thus wool fabrics might not need to be coated with N-doped TiO2 nanoparticles to realize its self-cleaning effect under visible light. Such important conclusions would provide wool materials with wide applications in clothing and technical products such as wastewater treatment. (C) 2014 Elsevier B.V. All rights reserved.