Mg(OH)2 (MH) nanoparticles were synthesized by hydration of the light-burned MgO at low temperature (70°C) with the additive of magnesium acetate. Mg(OH)2 nanoparticles have platelet-like structure and approximately 20-40 nm in thicknesses. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were applied to characterize the crystal phase, size distribution and morphology of the crystal particles. The effect mechanism of magnesium acetate was proposed