Fine BaTiO3 nanoparticles were prepared by hydrothermal synthesis under supercritical condition (400 degrees C and 30 MPa) from mixture of barium hydroxide and titanium dioxide as starting precursors. First, conditions for synthesizing BaTiO3 were examined by using batch reactors. High pH condition, pH > 13, is necessary to obtain phase pure BaTiO3. The reason was discussed based on the solubility of titanium dioxide, which that dissolution-recrystallization process is essential for the synthesis of BaTiO3 nanoparticles. Rapid heating of the starting precursors by mixing with high temperature water in a flow reactor is effective to synthesize smaller size and narrower particle size distribution for the BaTiO3 nanoparticles, compared with the case of slow heating with a batch reactor. (c) 2007 Elsevier B.V. All rights reserved.