Structural changes in anatase phase in four TiO(2) photocatalysts with annealing at high temperatures were followed by evaluating crystallite size and lattice strain of anatase phase separately and measuring the content of anatase. The rate constant k for the decomposition of methylene blue in its aqueous solution under UV irradiation was determined as a measure of photocatalytic activity. Marked dependences in crystallinity improvement, i.e., the growth of crystallite and the decrease in lattice strain, and in phase transformation from anatase to rutile phases of TiO(2) on annealing temperature was observed above 500 degrees C, depending on starting photocatalysts used. The phase transformation to rutile started after reaching of crystallite size to about 32 nm and of lattice strain to about 0.5 x 10(-3). Rate constant k was found to depend on both crystallite size and lattice strain of anatase; it increased with increasing crystallite size up to about 32 nm and decreasing lattice strain down to about 0.5 x 10(-3). Further increase in crystallite size and decrease in lattice strain induced the decrease in rate constant k, mainly due to the partial transformation of anatase to rutile. The present results showed that the activity of the photocatalysts was possible to be improved by annealing at a high temperature, by selecting an optimal condition of annealing for getting a high crystallinity in anatase phase and no phase transformation to rutile phase.