In the preparation of fine BaTiO₃ powders under hydrothermal conditions, the reaction mechanism was interpreted through solid-state kinetic analysis of the Johnson-Mehl-Avrami plot. In this experiment reactants were dissolved and consumed to spherical particles of 50 nms from aggregation of several nanometer-sized particles. The particulate formation of BaTiO₃ underwent a 1st-order hydrolysis-condensation reaction with phase-boundary transition in the early stage of the reaction regardless of the initial concentration of the feedstock. However, as the concentration of nutrients was reaction, dissolution followed by precipitation became dominant, and a diffusion-controlled reaction proceeded. When the concentration of nutrients was reduced to an extent that was not high enough to sustain supersaturation, the reaction was controlled by solidification for encapsulation of aggregated particles, inside of which the diffusion-controlled reaction slowly proceeded.