Zirconia nanocrystals were prepared by hydrothermal reaction of 0.05 M zirconyl nitrate and zirconyl acetate solutions at supercritical conditions of 400 °C and30 MPa for 1.8 s reaction time. Characterization of products were performed byx-ray diffraction, transmission electron microscopy, and Brunauer–Emmett–Teller measurements. The product particles were compared with zirconia particles prepared by conventional hydrothermal synthesis routes and precipitation-calcination. From the results, zirconia powders prepared in supercritical water had higher crystallinity than those obtained by other methods. Product particles with tetragonal crystal structure with a mean diameter of 6.8 nm could be formed from 0.05 M zirconyl acetate solution in the presence of 0.1 M potassium hydroxide at supercritical conditions.