Vapor-phase synthesis of ultrafine TiO2 particles was investigated experimentally in the simultaneous hydrolysis and oxidation of TiCl4. Experiments were made for the control of particle size, size distribution and phase composition of ultrafine TiO2 particles with the preheating of reactants. The molar ratio of H2O/TiCl4 was adopted as a variable for the control of particle size and size distribution in various reaction systems such as TiCl4–H2O, TiCl4–H2O–O2, and TiCl4–H2O–air. As the molar ratio of H2O/TiCl4 increased from 2 to 12, the average particle size decreased from 45 nm to 19 nm, and the rutile content of TiO2 powders increased from 21% to 54% in the TiCl4–H2O system. Changes in the average particle size were the same in the TiCl4–H2O–O2 system as in the TiCl4–H2O system, but particle-size distributions were narrower in the TiCl4–H2O–O2 system. The same tendencies of average particle size, size distribution and rutile content were also found in the TiCl4–H2O–air system. A high production rate of ultrafine TiO2 particles was obtained at the condition of high mole fraction of TiCl4 and low gas-flow rate in the presence of water vapor and preheating of reactants.