Using the residence time calculated by computer simulation for temperature and gas velocity distribution in CVC reactor, the kinetics on the formation of $TiO_2$ nano powder was analyzed for coagulation process, After abrupt increase of particle size at initial growth stage (< 0.2 $mus$ ), the particle grew in proportion of cubic root to time. The numerically calculated particle sizes well agreed with the experimental results. However, the coarse rutile $TiO_2$ powders having the particle size of over 40 nm were formed on the surface of quratz rod in the reactor. it is thought that the fine anatase particles condensed on quratz rod were sintered in a heated CVC reactor to grow and transform to coarse rutile phase, and the critical size for phase transformation anstase-to-rutile was around 25 nm tn this study.