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Sn 4+ -doped TiO 2 nanoparticles have been prepared by sol-gel method and annealed at different temperatures. Doping mode and existing states of Sn 4+ dopants as well as the constituents and phase transition of the resultant nanoparticles have been investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and high resolution transmission electron microscopy techniques. Under a low-temperature annealing (300 °C), the majority of Sn 4+ dopants are connected with Clions, forming -Sn-Cl x at the sample surface, while the others are doped into TiO 2 lattice in substitutional mode. The amount of substitutional Sn 4+ dopants decreases with the increasing annealing temperature. No -Sn-Cl x is observed at 450 °C. When the annealing temperature is high enough, Sn 4+ ions can replace lattice Ti 4+ ions no matter whether the crystal structure of TiO 2 is anatase or rutile. Furthermore, rutile SnO 2 crystal evolves with the increasing annealing temperature, which can act as seeds for the growth of rutile TiO 2 . Accordingly, the phase transition temperature from anatase to rutile of TiO 2 decreases obviously. This may help prepare Sn-doped TiO 2 materials with high photoelectric properties that can be used in many fields, such as photocatalysis and solar cell.