Nanometer rutile titania particles were synthesized by the gas-phase oxidation of titanium tetrachloride and aluminum trichloride in a high temperature gas phase reactor. The effect of oxygen preheating temperature TO(2), nitrogen flow rate QW into reactor tail, reaction temperature T and residence time t on TiO2 particle size was studied. The result showed that the enhancement of oxygen preheating temperature and nitrogen flow rate into reactor tail is benefit for controlling the particle size, and the TiO2 size increases with increasing reaction temperature or residence time. When t=1.73 s, T=1373 K, TO(2)=1143 K, QW=0.036 m3/h, and Xinlet =0.25, the TiO2 particles synthesized is pure rutile phase, and the size distribution is from 30 nm to 50 nm.