Nanoparticles of barium titanate were prepared by plasma chemical vapor deposition using an inductively coupled plasma technique that has a high potential for preparing nanoparticles. The present paper describes the details of the reaction control achieved by accelerating the reactions between Ba atoms and oxygen using direct injection of oxygen into the tail of the plasma flame. According to our previous reports, the crystalline phases of the powder products were different at each collection area in the CVD apparatus. This was due to the remaining active Ba atoms without a reaction in the plasma tail flame, as observed from the optical emission spectroscopy. After the reactions between Ba atoms and oxygen occurred by direct oxygen injection, BaTiO3 perovskite phases were produced as a major phase in each collection area. The optimal oxygen injecting conditions for obtaining perovskite BaTiO3 single phase are summarized as follows: the temperature of the plasma tail flame at oxygen-injecting position = approximately 1000 K and molar ratio of oxygen to the reactants (O-2/(Ba+Ti)) = 4000. Furthermore, BaTiO3 nanoparticles with average particle size under 10 nm were prepared by oxygen injection (the average particle size without oxygen injection is 15.4 nm). Well-crystallized BaTiO3 nanoparticles with spherical shape were observed by TEM.