We investigated a method in which magnesium hydroxide nanoparticles were synthesized by irradiation of microwaves with a hydrothermal method. The precursor was prepared with different molar ratios of magnesium sulfate and sodium hydroxide in an aqueous solution. In this experiment, the influences of the concentration of the precursor, pH, reaction watt and irradiation of microwaves on the magnesium hydroxide crystals were examined by the morphology and particle sizes of the products. We found that the average particle sizes of the magnesium hydroxide nanoparticles were 100-300 nm and they could be varied by a reaction pH of 10-13. Mg(OH)₂decomposed into MgO at a temperature range from 358 to 383℃.The magnesium hydroxide was synthesized within 15 minutes and showed a flake-shape morphology with a hexagonal phase. The nano particles were characterized by x-ray diffractometer (XRD), fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), scanning electron microscope (SEM) and particle size analysis (PSA).