To obtain inpurity-free and nm-sized barium titanate (BaTiO3) particles, a new 2-step thermal decomposition method from barium titanyl oxalates (BaTiO(C2O4)(2) . 4H(2)O) was proposed. At the 1st step, BaTiO(C2O4)(2) . 4H(2)O was annealed at 400 degreesC for 1 h in the O-2 flow. The annealing temperature of 400 degreesC was chosen for the following reasons; (1) no formation of BaCO3 and TiO2 and (2) the complete removal of H2O and other carbon species. This compound obtained at 400 degreesC was amorphous phase, and its chemical composition was BaCO3-TiO2. When this compound was annealed in air at higher temperatures, the large BaTiO3 particles were prepared with by-products such as BaCO3. Thus, at the 2nd step, to prevent the crystal growth and the formation of BaCO3, this compound was annealed above 600 degreesC in vacuum. Finally, the BaTiO3 single crystals with a size with 16.5 nm were prepared around 620 degreesC. These BaTiO3 fine particles were characterized using various methods to investigate defects and impurities in the particles. As a result, it was confirmed that there was no impurity in the BaTiO3 lattices. (C) 2003 Kluwer Academic Publishers.