Highly crystalline BaTiO3 nanoparticle was synthesized rapidly by hydrothermal reaction in supercritical water using a continuous flow reactor. The reactants of TiO2 sol (or TiCl4)/Ba(NO3)(2) mixed solution and KOH solution were used as starting materials and that was heated quickly up to 400 degrees C under the pressure of 30 MPa for 8 ms as reaction time. The XRD results revealed that the crystal phase of the obtained particles was cubic BaTiO3, indicating that the hydrothermal reaction in supercritical water was successfully proceeded under present reaction conditions. Primarily particle size of the BaTiO3 nanoparticle was determined by means of BET surface area, as small as less than 10 nm with decreasing the reaction pH. In contrast, dispersed particle size in solution measured by DLS (dynamic light scattering) technique decreased from 260 to 90 nm with increasing the reactants concentration. Aggregation of BaTiO3 nanoparticles might be depressed in the presence of coexisting nitrate anions. (C) 2010 Elsevier B.V. All rights reserved.