Nano-sized barium titanate powders were prepared by the high-gravity reactive precipitation (HGRP)-hydrothermal method. The properties and defects of hydrothermal barium titanate crystallites were investigated by TEM, XRD and FTIR. The mean particle size of the hydrothermal barium titanate was about 70 nm with narrow particle size distribution. The crystallite phases and OH defects were focused. The results show that the powders without heat treatment were crystallized as cubic-BaTiO3 and the absence of tetragonal with increasing calcined temperature to 1100°C. The IR results exhibited the OH defect content in the crystallites was to increase the heat treatment temperature and reduced the probability of forming barium vacancies.