The process of preparing magnesium hydroxide flame retardants with particular morphologies after hydrothermal treatment in the presence of organic modifier ethanediamine was mainly investigated. Influences of such factors as slurry concentration, modifier solution concentration and reaction temperature were studied. Meanwhile, the hydrothermally treated results were characterized by XRD, SEM, FTIR and TGA. The results indicate that when the reaction temperature was 180°C, and the reaction time ranges from 4 to 8 hours, the magnesium hydroxide and organic modifier solution concentration separately was 5% and 4 mol/L, a spherical crystalline magnesium hydroxide that has a symmetrical and regular crystal morphology was prepared. Such magnesium hydroxide materials were found to give an enhanced powder filtration.