Anatase TiO2 particles with an average size of 2.7 nm in an as-grown state were obtained below 373 K from an H2O-EtOH mixed solution of TiOSO4 by a facile liquid phase method. The volume ratio of H2O to EtOH was revealed to be the key for crystallinity of the particles, The as-grown particles prepared from the H2O-EtOH solution (H2O/EtOH (mL/mL) = 150/100) showed a high photoreactivity in the TiO2 photoinduced decarboxylation of HCOOH under a deaerated condition. In contrast, under an aerated condition, the rate of the reaction increased with an increase in calcining temperature (T-c) due to the improvement of anatase crystallinity, going through a maximum at T-c = 873 K.