Abstract: Nanometric magnesium oxide was prepared by hydrothermal method using magnesium chloride and ammonia water as the raw materials and polyethylene glycol as the dispersant. The crystal structure, microstructure, and particle size of the products were characterized by X-ray diffraction and scanning electron microscopy. The effects of molar ratio of MgCl₂ to ammonia, reaction temperature, and reaction time on the precursors were discussed, and the influences of calcination temperature and calcination time on the nanometric magnesium oxide were investigated by the differential thermal analysis of the precursors. The results show that, when the molar ratio of MgCl₂ to ammonia is 1.0:2.5, the reaction temperature is 200 ℃, the reaction time is 3.0 h, the precursor calcination temperature is 600 ℃, and the calcination time is 2.0 h, the MgO nanoparticles obtained are found to be disc-like and uniformly dispersed without agglomeration, showing a particle diameter of about 100 nm and a thickness of about 10 nm.