Abstract: Nano-zirconia (ZrO₂) is widely utilized in catalysis, ceramics, and functional materials due to its excellent physicochemical properties, while its performance is largely influenced by crystal phase and grain size. In this study, nano-zirconia was synthesized hydrothermally using zirconyl chloride (ZrOCl₂·8H₂O) and ammonia solution (NH₃·H₂O) as the precursors, with sodium hydroxide (NaOH) employed as a mineralizer. The effects of NaOH concentration and hydrothermal parameters (temperature and time) on the crystal phase, morphology, and particle-size distribution of the products were systematically investigated by X-ray diffraction, laser particle size analysis, and transmission electron microscopy (TEM). The results revealed that at the NaOH concentrations ranging from 0.1 to 0.5 mol/L, pure tetragonal zirconia (t-ZrO₂) with high crystallinity and relatively large crystallite sizes (10.4~15.3 nm) was obtained. When the NaOH concentration was increased to 1 mol/L, the proportion of monoclinic zirconia (m-ZrO₂) increased significantly, accompanied by a reduction in crystallite size to 6.0 nm, and a decline in crystallinity. Furthermore, both increasing the hydrothermal temperature (160~220 °C) and prolonging the reaction time (4~10 h) facilitated the transformation from the tetragonal to the monoclinic phase. Under the optimized conditions (180 °C for 6 h), uniformly dispersed nano-zirconia powder with an average particle size of about 7.4 nm was successfully prepared.