Ultrafine barium titanate (BaTiO3) particles with cubic phase were synthesized by a hydrothermal conversion reaction. The direct thermal hydrolysis of titanium tetrachloride with perchloric acid had been used for producing nanosized TiO2 particles. The size and morphology of the TiO2 particles were controlled by the molar ratio of acid to water (H/Ti ratio) in the solutions. As the H/Ti ratio is 1, the morphology of TiO2 particles was distinct elongated particles with the major axis ca. 80 nm and the minor axis ca. 10 nm. These TiO2 particles were predominated in the rutile phase. These TiO2 particles were then used as a precursor to prepare BaTiO3 by the hydrothermal conversion in a barium hydroxide solution. The crystal structure of the BaTiO3 particles was cubic phase with a lattice constant of 4.072 nm (a-axis). The morphology of the particles was near-spheres with a size distribution of 20-50 nm. The reaction temperature and time did not influence the size and the morphology of BaTiO3 particles, but the lattice constant of a-axis shrunk slightly and the impurity of BaCO3 decreased. The Ba/Ti molar ratio did not influence the morphology and crystal structure, but the mean particle size decreased with increasing the Ba/Ti ratio. The size and morphology of the BaTiO3 particles were different from those of the precursor TiO2 particles, indicating a dissolution-precipitation mechanism for the conversion of TiO2 to BaTiO3. The particles were cubic phase if the calcination temperature was lower than 900 degreesC and were converted to tetragonal phase with the tetragonality (c/a) of 1.0105 after calcination at 1150 degreesC for 2 h in air. (C) 2004 Elsevier B.V All rights reserved.