A series of nano-sized Cu/TiO₂ with various Cu contents were prepared by incipient-wetness impregnation method using Cu(NO₃)₂ as the starting material. The catalysts were characterized by X-ray diffraction (XRD), high resolution-trans mission electron microscopy (HRTEM), UV-vis, and X-ray photoelectron spectroscopy (XPS). Photocatalytic degradation of methylene blue in water was carried out under either UV light or visible light irradiation with very low intensities of light sources. Copper cations were incorporated into the TiO₂ surface structure. In XRD patterns, anatase and rutile phases were observed for all of the samples. No characteristic peak of copper oxide was found, implying that Cu ions were incorporated into TiO₂ lattice or copper oxides were highly dispersed and very small. With increasing copper concentration, the band gap energy of Cu/TiO₂ decreased. This change inferred that the Cu-doped TiO₂ had the trapping level and defect centers created by the substitution of Ti⁴⁺ ion by Cu²⁺ ion on the surface of TiO₂ lattice. Therefore, the lattice deformation affects the electronic structure causing the band gap shift. Moreover, doping copper would produce more Ti³⁺ ions that induce visible light response due to the creation of oxygen vacancies. It would induce more active hydroxyl radicals which are the main species for degradation of organic dye. The 0.5 wt.% Cu/TiO₂ showed the highest photocatalytic activity under both UV light and visible light irradiation for the degradation of methylene blue dye among all catalysts.