Due to its buffer property, activity, absorbability and safety, magnesium hydroxide is widely used in environmental protection, pharmaceutics, pulp and paper industry, and as flame-retardant filling in composite materials. The hydrothermal synthesis of magnesium hydroxide at low temperature (< 100°C) needs only simple equipment and is environmentally friendly and energy-saving, so the process is promising and easy for industrialization. In this study, we have investigated the effects of hexamethylenetetramine and ammonia additives on the hydrating synthesis of superfine Mg (OH)2 particles at 70°C. Mg (OH)2 particles formed were of platelet shape and the mean diameter was approximately 200nm. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were applied to characterize the crystal phase, size distribution and morphology of the crystal particles. Different morphologies of Mg (OH)2 particles were formed in hexamethylenetetramine and ammonia solutions. The generation velocity of hydroxyl (OH-) was found to play an important role in the distribution and morphology of Mg (OH)2 particles.