Carbon-doped TiO2 nanocrystalline was prepared by sol-gel auto-combustion method and characterized by X-ray diffraction, X-ray photoelectron spectra (XPS), Brunauer-Emmett-Teller method (BET), UV-vis diffuse reflectance spectroscopy (DRS). The analysis of (OH)-O-center dot radical formation on the sample surface under visible light irradiation was performed by fluorescence technique with using terephthalic acid, which readily reacted with (OH)-O-center dot radical to produce highly fluorescent product. 2-hydroxyterephthalic acids. It was found that the order of photocatalytic activity per unit surface area was the same as that of the formation rate of (OH)-O-center dot radicals unit surface area, namely, the greater the formation rate of (OH)-O-center dot radicals unit surface area was, the higher photocatalytic activity unit surface area was achieved, indicating that the photocatalytic activity unit surface area was positive correlation to the formation rate of (OH)-O-center dot radicals unit surface area over the catalysts. In this study, the optimum calcination temperature was 600 degrees C, at which the highest formation rate of OH radicals per unit surface area was, and thereby the highest photocatalytic activity per unit surface area was achieved. In addition, it could be found that the order of photocatalytic activity correlated very well with the amount of visible light absorption, namely, the stronger the visible light absorption, the higher the photocatalytic activity. (C) 2008 Elsevier B.V. All rights reserved.