Almost impurity-free, defect-free and dense barium titanate (BaTiO3) fine particles with various sizes from 20 to 430 nm were prepared by the 2-step thermal decomposition method and post-heating treatment. The crystal structures of these particles in the range from -150 to 150 °C were investigated using synchrotron radiation XRD measurement. The crystal structure refinement using Rietveld method revealed that BaTiO3 particles with the size over 40 nm were composed of two parts; (a) cubic shell and (b) core with successive phase transitions. The crystal structure of the core component can be related to the dielectric properties at room temperature. Moreover, it was revealed that the phase transition behavior of BaTiO3 nanoparticles was different from that of the BaTiO3 single crystal.