Nanosized barium titanate particles were prepared by high-gravity reactive precipitation (HGRP) method. The lattice structure, lattice strain, critical size, and hydroxyl defects of the BaTiO3 particles in the size range of 30-250 nm were investigated using SEM, XRD, Raman spectra, and TGA measurements. The average particle size of BaTiO3 powder formed at 80 degrees C was approximately 30nm with cubic paraelectric crystal structure. As annealing temperature increased, particle size increased (i.e. length of lattice constant decreased along a axis and increased along c axis). At the size of 70 nm, the crystal structure changed from cubic paraelectric to tetragonal ferroelectric phase. Large lattice expansions of more than 1.3% were detected in particles between 30 and 250 nm. The principles of the lattice expansion were discussed based on XRD, Raman spectra and TGA analysis. (c) 2005 Published by Elsevier B.V.