Barium titanate powders were synthesized via a hydrothermal process at 120-180 using titanium oxide powder with different particle sizes as starting materials in this study. When micron-sized titanium oxide was used, pure perovskite phase was not formed because of the insufficient chemical reactivity of titanium oxide. However, as nano-sized titanium oxide was utilized, pure barium titanate was obtained after the hydrothermal process was performed at 180 oC. The effects of sintering temperature on the dielectric properties of barium titanate were also investigated. Densified ceramics with high dielectric constants were obtained after sintering at 1200oC.