Stable sols with a homogeneous dispersion of ZnO nanoparticles were prepared by heating zinc hydroxide (Zn(OH)₂) precipitate in an ethylene glycol solution of zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) at 308 K. The formation of ZnO nanoparticles occurred within 1 h when the mixed solution of Zn(OH)₂ precipitate and 0.05 mol/L of the ethylene glycol solution of zinc nitrate hydrate was heated at 308 K. The obtained sol was homogeneous, and no aggregation of the ZnO nanoparticles was observed. The formation process of ZnO nanoparticles in the mixed solution of Zn(OH)₂ and the 0.05 mol/L ethylene glycol solution of zinc nitrate hexahydrate was examined by measuring changes in the UV-VIS spectra of the obtained sol and the XRD patterns of the particles separated from the sol. Strong absorption due to the electron transition between the band gap of ZnO appeared during heating at 308 K for 1 h. When the heating time decreased, the shift in the absorption edge to a shorter wavelength was observed. This shift in the absorption edge would be related to changes in the particle size during the formation process of ZnO nanoparticles. Furthermore, the photoluminescence spectra of the obtained ZnO nanoparticles were examined. The intensity of photoluminescence increased with increases in the concentrations of zinc nitrate hydrate in the ethylene glycol solution.