Barium titanate (BaTiO3) particles with average size of smaller than 100 nm were synthesized by a hydrothermal process at 100°C to 300°C. The c- to a-axis ratios of the particles increased as the synthesis temperature increased. Disk capacitors were fabricated with as-synthesized particles and some additives to obtain X7R capacitors. The temperature coefficient of capacitance with particles synthesized at 100°C was much larger than that for 300°C, indicating that the former had uniform composition in the grain while the latter had a core-shell structure. This data show that barium titanate particles hydrothermally synthesized at high temperature are useful when creating the X7R capacitors with small grain size.