Carbon nanotube/TiO2 composites were synthesized by hydrothermal method using titanic acid nanotubes as the TiO2 precursor. The crystalline structures, morphologies, light absorption properties, specific surface areas of the prepared powders were investigated with XRD, TEM, UV-Vis absorption spectra and N-2 adsorption-desorption. The photocatalytic activities of the composites were evaluated by monitoring the degradation rate of methylene blue (MB) under the visible light irradiation. The mechanism of the improved photocatalytic activity of the photocatalyst was discussed. It was found that all of the composites upon 24 h of hydrothermal reaction show the diffraction peaks indexed to anatase TiO2 and the critical reaction time for anatase TiO2 nanoparticles to form is 3 h. The visible light photocatalytic activities of the catalysts in this experiment were affected by a combination of BET specific surface area, MWCNT content and hydrothermal reaction time. (C) 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.