Abstract: Taking the magnesite as raw material, a process was developed to prepare magnesium hydroxide under atmospheric pressure. Optimum calcining conditions were determined through calcination tests at 600°C for 9hr, under which the magnesite can be fully decomposed with high chemical activity of the product magnesium oxide. When calcining at 700-750°C for 2-4 hr, the decomposition rate of magnesite can be maximized but the activity of magnesium oxide decreases somewhat. Then, taking the powdered magnesium oxide as raw material, the suitable synthesizing process was investigated for the preparation of magnesium hydroxide at normal temperature. The results showed that the powdered magnesium hydroxide with high filterability and purity and regular flaky hexagion crystal can be prepared via a hydro-thermal aging treatment at 80°C for 30 min done to the floccular magnesium hydroxide precipitate quickly synthesized. A conclusion is thus drawn that the hydro-thermal aging reaction is of importance to the preparation of magnesium hydroxide with regular crystal morphology.