Fine barium titanate crystallites with various Mg contents were prepared by a hydrothermal method. These particles were spherical single crystallites with an average size of about 63 nm, regardless of Mg contents from 0 to about 0.15 wt%. It was confirmed that Mg replaced substitutionally on Ti site. On the refinement of the crystal structure with a Rietveld method, the crystal structure of the particles without Mg was tetragonal with a tetragonality of 1.0015 while those including Mg approached from tetragonal to cubic gradually with increasing Mg content. On the other hand, a Raman measurement indicated that the local structure remained tetragonal regardless of Mg contents. Moreover, the resonance frequency for each phonon mode was independent of Mg contents while their damping factor increased with increasing Mg content, which suggested that state of the phonon became more unstable with increasing Mg content. Therefore, the lattice defects induced by Mg into the barium titanate particles can give the phonon a similar effect to that of temperature in barium titanate single crystal.