Crystalline BaTiO₃ powder of cubic phase, the mean particle site of which was 0.5 μm, was prepared by hydrothermal synthesis at temperature between 80 and 200 ℃ with KOH as a mineralizer. A mixed phase of anatase and amorphous was formed with 0.2M KOH, but a stable phase of BaTiO₃ crystal powder was obtained when the concentration of KOH was 0.8M and above. As the concentration of the mineralizer increased, the relative supersaturations of Ba (OH)⁺ and Ti(OH)_χ^4-χ were enhanced. A lot of crystal powder were, therefore, formed instantaneously due to an increase in nucleation rate, and consequently, the mean particle size was reduced and the size distribution became narrower. As the reaction temperature increased, condensation reaction between Ba(OH)⁺ and Ti(OH)_χ^4-χ was accelerated, which led to a decrease in hydroxide concentration, so that agglomeration of powder was abated. However, the particle size became larger due to an increase in the crystal growth rate.