Nano-TiO2 powders were synthesized by the solvothermal method under various reaction conditions in order to obtain average crystallite sizes of 9-15 nm. The amounts of surface defect of TiO2 were measured by means of temperature-programmed desorption of CO2 and electron spin resonance spectroscopy. It was found that the ratios of surface defect/specific surface area increased significantly with increasing TiO2 crystallite size. The TiO2 with higher amounts of surface defects exhibited much higher photocatalytic activity for ethylene decomposition. (c) 2006 Elsevier B.V. All rights reserved.