Mg(OH)(2) was pptd. from aq. solns. of Mg salts with NH4OH at 20-80 degrees C, studied for particle size distribution, wettability, dispersive and adsorptive properties and crystal structure and then used as filler of polypropylene-matrix composites. The Mg(OH)(2) nano particles produced by conversion of MgSO4 at 60 degrees C showed an improved Young modulus and nonflammability.