A new method for ultrafine barium titanate (BaTiO3) particles with diameters of around 5 nm is proposed. In this method, barium titanyl oxalate aqua solution with low concentration below 10-3 mol/l was used as the starting material. The droplets with a size below 3 μm were atomized with an ultrasonic vibrator, dried and thermally decomposed at higher temperatures over 300°C. In the preparation of the BaTiO3 particles, there were two parameters such as thermal decomposition temperature and precursor solution concentration. Therefore, various particles were prepared by changing these parameters. Finally, non-aggregated nm-sized BaTiO3 particles with an average diameter of 5.2 nm, despite wide size distribution from 2 to 20 nm, were prepared by using the precursor solution with 10-6 mol/l.