Nanometer-sized titania particles were prepared by low thermal of vapor hydrolysis process titanium tetrachloride (TiCL4) in a furnace type aerosol reactor. Reaction model was introduced because simple reaction coagulation (SRC) model, which is valid at high conversion, was found to be improper at high concentration and low conversion. The size of titania particles increased from 10 to 50 nrn with increasing input concentration of TiCl4 from 3.06 x 10 exponent -7 to 1.59 x 10 exponent -6 mol/ml and the residence times from 7s to 35s at 250°C. The reaction model predicted the decreasing trend of the particle size with increasing temperature, whereas the SRC model simulation predicted that the size was an increasing function of the temperature.