A simple and rapid technique was established for synthesis of ZrO₂ nanoparticles from zirconium ethoxide (Zr(OEt)₄) and zirconium hydroxide (Zr(OH)₄) as the precursors, using the sub- or supercritical hydrothermal method. The precursors were treated in a batch-type reactor with the reaction temperatures between 200 and 500 °C for 10 min. The products obtained at temperatures higher than 300 °C had mixtures of tetragonal and monoclinic phases. The higher the reaction temperature, the higher the volume fraction of the monoclinic phase. Selection of a suitable precursor is the most important point in this synthesis technique. Zr(OH)₄ was found to be a better precursor for obtaining ZrO₂ with a higher volume fraction of monoclinic phase than Zr(OEt)₄ under the same synthesis conditions. Single phase pure monoclinic ZrO₂ was successfully obtained after heat treatment at 500 °C. The growth rate of ZrO₂ nanoparticles obtained from Zr(OEt)₄ and Zr(OH)₄ was different. The products contained some chemisorbed water and/or a hydroxide group on the surface. Their presence was dependent on the reaction temperature and the particle size, and not on the precursor used. However, there was no significant difference among the band gaps of ZrO₂ synthesized from different precursors, confirming that the type of precursor does not affect the quality as much as the crystallinity of the products.