Barium strontium titanate was obtained by hydrothermal treatment of barium strontium titanate citric precursor solution, previously prepared by complex polymerization method. The thermally induced phase evolution was followed at various temperatures up to 800 °C using thermogravimetric and differential thermal analysis, X-ray diffraction analysis, and Raman spectroscopy. Microstructural characterization of barium strontium titanate powders was performed by scanning and transmission electron microscopy. The proposed synthesis route has been proven as a better and faster method for barium strontium titanate powder preparation as compared to the conventional complex polymerization route. The method was found efficient for production of low agglomerated, fine, nanosized barium strontium titanate powder with well defined stoichiometry, and sub-micron particle size. The results of structural and microstructural characterization showed the complete crystallization of carbonate-free barium strontium titanate powder at 700 °C with an average size of crystallites below 50 nm.