Nanosized TiO2 particles were prepared by the hydrothermal method from the amorphous powders which were precipitated in an aqueous peroxotitanate solution. The physical properties of the nanosized TiO2 particles prepared were investigated. We also examined the activity of TiO2 particles as a photocatalyst on the decomposition of orange II. The titania sol can be successfully crystallized to the anatase phase through hydrothermal aging at temperatures higher than 160 degrees C. The particle size increases from 18 to 26 nm as the synthesis temperature increases from 140 to 200 degrees C. Titania particles prepared at 180 degrees C show the highest activity, and titania particles calcined at 400 degrees C show also the highest activity on the photocatalytic decomposition of orange II.