To understand the mechanism by which titania (TiO2) nanoparticles are generated by a CVD method, the primary nucleation mode size distributions of TiO2 nanoparticles prepared from two different chemical precursors (TTIP and TiCl4) were directly measured using a DMA/PSM/CNC system. This represents the first report of such direct measurement in the nucleation mode. These results are particularly important for experimental investigation of particle nucleation and growth, something which has never been achieved before. In the nucleation mode, titania nanoparticles with a diameter of about 2 nm were produced by nucleation. At low reactor temperatures, nucleation and surface reaction were major contributors to particle generation. At a high reaction temperature, coagulation and sintering became more important. The morphology and crystallinity of the particles were investigated by TEM and XRD as a function of temperature and precursor concentration. The properties of the titania nanoparticles, such as particle size distribution, the morphology and crystallinity, changed as a function of reaction temperature and chemical reaction rate.