Zirconia nanopowders were prepared in the ultrasonic field by the sol-gel method and the sonochemical effect on the structure of zirconium hydroxide and the zirconia nanopowder properties were systematically investigated in this work. Ultrasound was introduced into the different stages of the synthesis of zirconia nanopowders in sol-gel reaction system, and zirconium hydroxides and the zirconia nanopowders with different properties were obtained. The results indicated that ultrasonic cavitation could not only disaggregate the agglomerates of zirconia colloidal particles but also reduce the amount of coordinated H2O, free H2O and free hydroxyl groups of the zirconium hydroxide colloidal particles, thus effectively preventing the formation of hard agglomerates in zirconia powders. Moreover, the effects of different ultrasonic output powers and treatment cycles on the structure and properties of ZrO2 nanopowders were studied by TEM, XRD and SAXS. Zirconia nanopowders with an extremely small crystallite size (10.3 nm) and a narrow size distribution were yielded with 520 W ultrasound for 6 treatment cycles on the formation period and 600 W ultrasound for 2 treatment cycles on the washing period. It is concluded that the ultrasonic field is a potential method for nanopowder preparation.