In preparing BaTiO₃ thin films under hydrothermal conditions, the effects of concentrations of nutrient and mineralizer, and reaction time on crystallinity, grain size, surface roughness, and film thickness were investigated. Experiments were performed in the ranges of 0.1-1.5 M BaCl₂·2H₂O or Ba(OH)₂·8H₂O and 0.1-1.5 M KOH with varying reaction time from 0.16 to 8 hours at 140℃. Bimodal dispersion of crystalline grains on the surface of BaTiO₃ thin films was predicted through nucleation and crystal growth reaction. As the concentrations of nutrient and/or mineralizer increased, grain size of the thin film became smaller, but more uniform and compact. When 0.4M Ba(OH)₂·8H₂ was used with 1.0 M KOH, a reaction time longer than 4 hours was required in order to fabricate BaTiO₃ thin films.