Highly crystalline BaTiO3 nanoparticle was synthesized rapidly by hydrothermal reaction in supercritical water using a continuous flow reactor. The reactant solution of 0.05 M TiO2 sol and 0.06 M Ba(OH)(2) was used as starting materials and that was heated quickly up to 400 degrees C under the pressure of 30 MPa with in as short as 2 s to 7 ms reaction rime. The X-ray diffraction (XRD) results revealed that the crystal phases of the obtained particles were only BaTiO3, indicating that the hydrothermal reaction in supercritical water was successfully proceeded under present reaction conditions. Particle size of the BaTiO3 nanoparticle was determined by means of BET surface area, as small as less than 10 nm with reducing the reaction time from 2 s to 7 ms. Furthermore, high-resolution transmission electron microscopy (TEM) image of BaTiO3 nanoparticles showed highly crystalline nature of BaTiO3, even if the time for crystallization of BaTiO3 was extremely reduced. The formation of highly crystalline BaTiO3 nanoparticles by the short reaction period is thought to be strongly attributed to the properties of supercritical water. Especially, lower dielectric constant property of supercritical water expected to accelerate the hydrothermal reaction of BaTiO3, and allowed the synthesizing of BaTiO3 nanoparticles only in 7 ms. (C) 2008 Elsevier B.V. All rights reserved.