Size and morphology transformation as well as the crystallization path of monodisperse titanium dioxide particles were studied in aerosol flow reactor. Solid, hydrated titanium oxide particles were prepared from titanium alkoxide by a droplet hydrolysis and a subsequent size classification. The particles were carried to a reactor in air at temperature range of 20–1500°C. The inlet particle size of 100 and gradually decreased 40% with increasing temperature as investigated by a tandem differential mobility analyzer (TDMA) system. The decrease was due to decomposition of the hydrated Ti–O particles, densification of the forming TiO2 particles, and a phase change from anatase to rutile. Above 1000°C the mobility size increased due to crystal-habit formation that created faceted, elongated particles and consequently an increased dynamic shape factor. Microstructure investigations with a transmission electron microscope (TEM) showed that nanocrystalline anatase particles were present at 600°C and transformed to rutile up to 1100°C with simultaneous crystallite growth. Single-crystal rutile particles were observed at temperatures with the increased mobility diameters.