Four kinds of magnesium hydroxide (Mg(OH)(2)) with different particle sizes are chosen and mixed with ethylene vinyl acetate copolymer (EVA) to investigate the effect of particle size on the flame retardancy of composites, which is evaluated by limiting oxygen index (LOI) testing, horizontal fire testing, and cone calorimeter. When Mg(OH)(2) filling level changes from 35 to 70 wt %, the composites filled with nano-Mg(OH)(2) do not always possess the best flame retardancy, and among the composites filled with micro-Mg(OH)(2), the composites filled with 800 mesh Mg(OH), show the best flame retardancy; however, the composites filled with 1250 mesh presents the worst one. So the effect of particle size on the flame retardancy of microMg(OH)(2)-filled EVA is not linear as expected. All the differences are thought to result from both particle size effect and distributive dispersion level of Mg(OH)(2). To prepare the composites with better mechanical properties and flame retardancy, authors suggested that Mg(OH), of smaller size should be chosen as flame retardant, and good dispersion of Mg(OH), particles also should be assured. (c) 2006 Wiley Periodicals, Inc, J Appl Polym Sci 100: 4461-4469, 2006.