The flame retardancy and mechanical properties of ethylene vinyl acetate (EVA) polymer nanocomposite based on magnesium hydroxide (MH) nanoparticles with lamellar-shape morphological structures and synergistic agent microcapsulated red phosphorus (MRP) have been studied by limiting oxygen index (LOI), cone calorimeter test (CCT), UL-94 test, tensile strength (TS), and elongation at break (EB). Results showed that LOI values of lamellar-like nanosized MH (50 x 350 nm(2)) samples were 1-7 vol. % higher than those of the common micrometer grade MH (1-2 mu m) in all additive levels. When 1-3 phr MRP substituted for nanosized MH filler, LOI value increased greatly from original 37 to 55, and met the V-0 rating in the UL-94 test. The values of TS for MH nanoparticles composites increased from 10.4 to 17.0 MPa as additive loading levels increased from 80 to 150 phr, respectively, while the corresponding values for common micrometer MH composites decreased steadily from 9.7 to 7.1 MPa. Thermogravimetric analysis (TGA) and dynamic Fourier-transform infrared spectroscopy (FTIR) results revealed two-step flame-retardant mechanism. First, MH particles decompose endothermically with the release of 30.1% hydration water in the 320-370 degrees C temperature range. Second, MRP promote the formation of compact charred layers slowly in the condensed phase in the 450-550 degrees C temperature range. (c) 2007 Wiley Periodicals, Inc.