A novel sonochemical method to directly prepare anatase nanocrystalline TiO 2 has been established. TiO 2 particles were synthesized by the hydrolysis of titanium tetraisopropyl in the presence of water and ethanol under high-intensity ultrasonic irradiation (20 KHz, 100 W/cm 2 ) at 90 o C for 3 h. The product structure and the particle sizes were dependent upon the reaction temperature, the acidity of the medium and the reaction time. Characterization was accomplished using various different techniques, such as powder X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetry-differential thermal analysis (TG-DTA), and Fourier transform infrared (FT-IR) spectroscopy. The TEM images showed that the particles of TiO 2 were columnar in shape and the average sizes were ~3x7 nm. The formation mechanism of nanocrystalline TiO 2 under high-intensity ultrasonic irradiation was also discussed.