Nanocrystalline titania powders were synthesized at low temperature (⩽100°C) by a sol-gel method that achieved fine control of particle size and polymorph fraction. X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV-Vis spectroscopy were used to characterize the phase assemblages, crystal size and band gap of the powders. It was demonstrated that larger, well-ordered titania crystals can be obtained by increasing aging temperature and time. These processing parameters can be adjusted to select specific polymorphs from the gel precursors with particular size and shape. The quantum size effect was observed in the size-controlled nanocrystalline titania particles, leading to a blue shift in UV absorption with decreasing in particle size. The anatase to rutile transformation, which may proceed with brookite as a transition phase, is dependent on both particle size and surface structure of the nascent crystals.