Barium titanate fine particles were prepared by a hydrothermal method using titanium tetrahydroxide gel and barium hydroxide. In particles with an average size of 89.62nm, the crystal structure changed from cubic to tetragonal accompanied with a decrease in the amount of lattice defects such as lattice hydroxyl groups, without a change in particle size. This result revealed that a physical quantity with size much smaller than the particle size could determine the crystal structure of barium titanate. To explain this phenomenon, we proposed a new model based on the stability of lattice vibrations, in which the amount of the lattice defects can determine the correlational size of barium titanate dipoles, and the tetragonality (c/a) is dependent on the correlational size. Moreover, the model can be used to explain the size effect on relative permittivity and Curie temperature in barium titanate.