The investigation was carried out to improve the precipitation process of magnesium hydroxide from seawater by ultrasound of high intensity. The purpose was to obtain magnesium hydroxide precipitate with particle size as small as possible, which would retain the same particle size after the drying process. The investigations were conducted on magnesium hydroxide samples in the form of sludge and voluminous powder. The chemical analysis indicates a high purity of obtained precipitate, i.e. high percent of MgO (65.45 mass. %) in magnesium hydroxide sample. The FTIR analysis confirmed the chemical analysis. The O-H stretching bonded with Mg indicates a sharp peak at 3699.77 cm -1 , while the bending appears at 1473.50 cm -1 . The O-H stretching bonded attached at the surface of the sample (sludge) indicates as a broad band at 3600-3200 cm -1 , while the bending appears at 1635.13 cm -1 . The mean particle size of magnesium hydroxide sludge (5.75 µm) and voluminous powder (7.58 µm) were determined by the laser scattering method. The application of the ultrasound affects the particle size and reduces of precipitated magnesium hydroxide agglomerates during the precipitation process and after drying. The deviations of particle size in samples of magnesium hydroxide in form of sludge and voluminous powder after application of high intensity ultrasound are result of forming twin structures. The freeze dry method has not been successful in the prevention of forming twin structures. The formations of flaky and filamentous structures of irregula r shapes (below 100 nm) agglomerated into much larger structures (μm) in samples of magnesium hydroxide voluminous powder were confirmed by SEM analysis. The filamentous structures are 1 – 10 nm in size, while the flakes are much larger.