TiO2 nanoparticles with crystallite size less than 15 nm were prepared from TiCl4 precursor by a modified sol-gel route. As-prepared TiO2 nanoparticles were extensively characterized using X-ray diffractometry, and their crystallite size was calculated using Debye-Scherrer formula. The phase composition and crystallite size of TiO2 nanoparticles were dependent on reaction parameters including TiCl4 concentration, reaction temperature, annealing temperature, and surfactant. The crystallite size increases with increasing annealing temperature. Amorphous-anatase phase transition occurs at 400-450oC and anatase-rutile phase transition occurs at ca. 700oC.