Photocatalytic activity of anatase powders was discussed in the relation to their crystallinity. Anatase powders were synthesized under hydrothermal condition at 180 degreesC from TiOSO4 aqueous solution with 0.1, 1 and 3 mol/l concentration and then annealed at high temperatures above 700 degreesC. Photocatalytic activity was evaluated as a rate constant of decomposition reaction of methyleneblue (MB) determined from the changes of relative concentration of MB with irradiation time. Crystallinity was measured from full width at half maximum intensity (FWHM) of 101 X-ray diffraction line of anatase phase. High rate constant for MB decomposition was found to obtain only at a narrow range of FWHM of diffraction line. Both larger FWHM of anatase phase and also smaller FWHM obtained by annealing at high temperatures are not desirable for high photoactivity. The decrease in rate constant at small FWHM side was supposed to be due to cooperative effects of partial transformation of anatase to rutile and decrease in surface area. Small rate constant for the samples with large FWHM may be attributed to high possibility of recombination of excited electrons and holes by the scattering at structural defects. (C) 2004 Elsevier B.V. All rights reserved.