Nanocrystalline TiO2 powders with high photocatalytic activity were prepared by one-step microemulsion-mediated hydrothermal method using tetrabutylorthotitanate (TiO(C₄H₉)₄, TBOT) as precursor. The as-prepared TiO₂ powders were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and the Brunauer-Emmett-Teller (BET) specific surface area measurements. The effects of the oil/water ratio and hydrothermal temperature on the microstructures and photocatalytic activity of the TiO₂ powders were investigated. The results suggest that increasing the oil/water emulsion ratio significantly decreased the particle size of the as-prepared TiO₂ powders and improved the photocatalytic activity. With hydrothermal temperature increasing, the average crystallite size increased and the photocatalytic activities of TiO₂ powders decreased.