Barium titanate fine particles of cubic system were synthesized hydrothermally from aqueous barium hydroxide, solutions with fine particles of either amorphous or crystalline (rutile) titania in suspension. The mean size of barium titanate particles prepared from amorphous titania ranged from 0.03 mu m to 0.11 mu m, depending on hydrothermal conditions. The particle size approximately agreed with the crystallite size (0.04-0.09 mu m). On the other hand, the mean size of barium titanate particles prepared from rutile titania ranged from 0.2 to 0.7 mu m, which was about six times as large as the crystallite size. The difference of sizes of barium titanate particles prepared might be ascribed to the difference in dissolution rates of amorphous or crystalline titania particles.