Monodisperse and spherical barium titanate (BaTiO3) powders with diameters of 200-470 nm were directly prepared by a low-temperature hydrothermal method at 90°C. Spherical titania (TiO2) powders, ranging in size from 150 to 420 nm, were initially prepared by a controlled hydrolysis and condensation reaction, aged in a highly alkaline solution for 12 h, and then hydrothermally reacted with barium hydroxide to be converted to BaTiO3 without a morphological change. The aging step of the TiO 2, where the surface of TiO2 was highly densified through elimination of the pores, was indispensable to retain the sizes and shapes of TiO2 in the resulting BaTiO3. This was due to the fact that the formation of BaTiO3 proceeded by an in situ reaction mechanism. The resulting BaTiO3 powders exhibited dense and nonporous structures even after calcination at 1000°C.