In this paper, magnesium hydroxide (Mg(OH)2) nanoparticles with tunable morphologies have been selectively synthesized via a facile template-free solvothermal route. The as-synthesized samples were characterized by instrumental analyses such as transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). It was found that the morphology and size of the final products strongly depended on the reaction temperature and duration time. By manipulating the temperature and time in the solvothermal treatment, fiber-like, sphere-like and garland-like magnesium hydroxide nanoparticles were obtained. The described method without any catalysts or templates for preparing magnesium hydroxide nanoparticles is promising for industrial production in competition with other approaches.