Nanoscale TiO2 particles were synthesized in the diffusion flame reactor from the titanium tetraisopropoxide (TTIP) decomposition reaction. The effects of O-2 and CH4 flow rates and the filter position in the reactor on the specific surface area, primary and secondary particle sizes, morphology, phase composition, and crystallite size of TiO2 particles were investigated by Brunauer-Emmett-Teller (BET), electrophoretic light scattering (ELS), transmission electron microscopy (TEM), and X-ray diffraction (XRD) techniques. The larger primary particles can be observed at the lower O-2 flow rate or the higher CH4 flow rate, while the secondary particle size increases at the higher O-2 flow rate or the lower CH4 flow rate. The degree of aggregation increases as the TiO2 particles are collected in a higher position on the filter. The transformation of anatase to rutile phase and the growth of anatase grain size in the TiO2 particles are promoted as the flame temperature increases.