Hafnium oxide nanoparticles (NPs) were synthesized by a hydrothermal route, using hafnium tetrachloride (HfCl₄) as the starting material and sodium hydroxide (NaOH) to adjust the pH. Through changing the aging temperature, concentration of NaOH and reaction time, both pure tetragonal hafnium oxide (t-HfO₂) and pure monoclinic hafnium oxide (m-HfO₂) were obtained. X-ray diffraction (XRD) spectra and transmission electron microscopy (TEM) images indicated that the shapes of t-HfO₂ NPs and m-HfO₂ NPs were near-spherical and spindle-like, respectively. The formation of t-HfO₂ NPs or m-HfO₂ NPs is probably related to their crystal cell structure, thermodynamic and kinetic stabilities. Tetragonal HfO₂ is produced originally in the process of the formation of monoclinic HfO₂. A higher temperature, lower concentration of NaOH, longer reaction time and addition of m-HfO₂ seeds are beneficial for the formation of m-HfO₂ NPs. By analysis and calculation of the equilibrium constants involving hydrolysis of hafnium ions, the changes in the mole fractions of hafnium hydro-complexes with pH were determined. The Hf(OH)₆²⁻ ion is assigned to the precursory hydro-complex for the formation of HfO₂ nanoparticles transformed from Hf(OH)₄ gel according to a comparison between the influences of pH on the equilibrium and the formation of HfO₂ particles. Moreover, the formation of HfO₂ NPs was obviously promoted and the size was reduced by addition of seeds, suggesting that the formation of HfO₂ NPs is controlled by the surface-deposition reaction. The above results are of great importance for studying nano-inorganic solution chemistry.