Monodisperse BaTiO3 microspheres were synthesized using Ba(OH)(2), a homemade amorphous TiO2 precursor, and dissolved CO2 as the raw materials. To dissolve sufficient CO2 in the mixing solution, the reaction process was carried out in air. Then, a BaCO3 layer, as a fixed barium source, was coated on the surface of the TiO2 precursor template through a hydrothermal process at 90 degrees C. Furthermore, the BaCO3 reacted with the TiO2 precursor to form pure BaTiO3 through in situ conversion during subsequent heat treatment at 800 degrees C. Compared to the commonly used acid soaking method for the removal of BaCO3 after hydrothermal treatment, the method combining in-air mixing with a low hydrothermal temperature and heat treatment yielded BaTiO3 microspheres with a more monodisperse homogenous morphology, which have a greater potential in various applications.