The pure $TiO_2$ particles have been prepared in vapor-phase hydrolysis of titanium tetraisopropoxide(TTIP). The rate of TTIP hydrolysis was so fast that the overall rate of formation of $TiO_2$ was controlled by the rate of mixing of TTIP and $H_2O$. Thus, the primary $TiO_2$ particles were prepared in nano sizes to form chainlike aggregates due to rapid coagulation. The pure $TiO_2$ particles as prepared were amorphous at the reactor set temperatures below $400^{circ}C$ and became anatase at the temperatures of $450^{circ}C$ above while the weak rutile peaks were also observed above $800^{circ}C$. The actual size of primary particles as prepared were reduced by increasing the reactor set temperature while their crystalline sizes as well as BET sizes increased by post-sintering.