Barium titanate ultrafine particles were synthesized from amorphous titania by a hydrothermal method. The mean size-of the barium titanate particles prepared at a hydrothermal treatment time of 4 h, was nearly equal to 0.04 5 mu m in the range of barium-to-titanium molar ratio (BT) greater than or equal to 2, and approximately agreed well with the crystallite size. At a BT molar ratio of 1.0, the mean particle size increased to 0.2 mu m, while the crystallite size remained constant at 0.045 mu m. When the particle size ranged from 0.12-0.20 mu m, prepared for the BT molar ratio of 1.0-1.4, the specific dielectric constant for a sintered disc composed of these particles attained a value of 5000 or more. As the BT molar ratio increased to exceed 1.5, when the mean particle size decreased from 0.13 mu m to 0.045 mu m, the specific dielectric constant for the sintered disc was decreased greatly. The specific dielectric constant for the sintered disc can be correlated well with the size of the composing particles.