In this paper, TiO2-10 wt.% CNT nanocomposite powder doped with 1 at% Cu was synthesized by sol-gel method and its application for the degradation of organic pollutant in aqueous solution was examined in the visible light region. The samples were characterized by X-ray diffraction, field emission scanning electron microscopy, differential thermal analysis, surface area analysis, diffuse reflectance spectroscopy, photoluminescence spectroscopy and ultraviolet-visible spectrophotometer. The effect of CNT and doped Cu on the phase transformation during the calcination of as-synthesized amorphous TiO2 was studied in detail. A coating of anatase phase TiO2 with the thickness of 80 nm on the well dispersed CNTs was observed in the microstructure of calcined nanopowder at 440 degrees C. Also, addition of CNTs to the TiO2 led to a decrease in band gap energy from 3.2 to 3.05 eV and more decrease to 2.85 eV by both addition of CNT and Cu doping to TiO2. The effect of main parameters of photocatalytic dosage, irradiation time and MB concentration were investigated on methylene blue degradation efficiency and optimal conditions were proposed. The Cu doped TiO2-CNT catalyst exhibited better photocatalytic activity for methylene blue degradation (81.5%) than the TiO2-CNT catalyst (62%) and pure TiO2 sample (5%) after 60 min visible light irradiation. The detailed mechanism to explain the enhancement of the photocatalytic properties was proposed, as well.