In this work sea bittern was used in order to obtain a high quality precipitate of magnesium hydroxide, i.e. magnesium hydroxide in the form of a high purity powder with a particle size as small as possible. Used sea bittern water was sampled at Solana Ramova from the Krvavica (Makarska). The investigations were carried out on magnesium hydroxide samples in the form of sludge and powder. Magnesium hydroxide in powder form was prepared with a temperature change to investigate the influence of temperature on the particle size of the precipitation obtained. The results of the chemical composition in terms of Mg and Ca indicate a high percentage of MgO (97.47%) in the sample, while the CaO content (2.89%) was significantly lower. The EDS analysis confirms the high purity of the magnesium hydroxide powder obtained. The analysis of the mean particle size distribution was determined before (sludge) and after drying (powder) of the samples by the laser light scattering method in the range of 20 nm to 2 mm. For the purpose of agglomerate reduction, the samples were analyzed immediately before the measurement for 100 s without and with ultrasound. The mean particle size of magnesium hydroxide sludge is 3.082 µm. The deviation of the particle size distribution in powder samples without (11.482 µm) and with temperature increase (6.607 µm) is visible. The morphology of magnesium hydroxide powder confirms the formation of flaky and filamentous structures of irregular shape (up to 400 nm thickness) agglomerated into much larger structures (µm). The filamentous structures are reduced to 100 nm in the powder sample by applying the temperature.