A novel nanomagnesium hydroxide powder and three kinds of micro-Mg(OH)(2), with different particle sizes, were chosen as fillers and mixed with ethylene-propylene-diene monomer rubber (EPDM) to form a series of Composites by a traditional rubber-processing technique. The results showed that the mechanical properties of composites improved with decreasing particle size. The nanocomposites were far stronger than the microcomposites, which also supported the view that rubber reinforcement requires nanoreinforcernent. The effect of particle size on the fire resistance of composites was investigated by cone calorimetry and limiting oxygen index analysis, which showed that the particle size Of powder had an impact on the fire resistance of composites. For the composites filled with untreated powder, the peak value of heat release rate decreased and T-ign increased with decreasing particle size. In conclusion, the fire resistance of nanocomposites was better than that of microcomposites. Surface modification of particles sometimes substantially improved the mechanical properties of nanocomposites, but had no effect on either the mechanical properties of microcomposites or the fire resistance of nanocomposites and flame retardance. (C) 2004 Wiley Periodicals, Inc.