Magnesium hydroxide (Mg(OH) 2 ) particles were synthesized and modified via a reaction-hydrothermal process using Ca(OH) 2 and MgCl 2 as precursors. The influence of KOH mineralizer on the hydrothermal modification of Mg(OH) 2 was investigated. X-ray diffraction, scanning electron microscopy, thermogravimetry and specific surface area analyses indicated that KOH played a key role in the recrystallization of Mg(OH) 2 . It was found that the morphology and dispersion of Mg(OH) 2 particles improved significantly with an increase of KOH concentration from 1 to 4mol/l.