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In this paper, nano-silica with and without surface modification was added into linear low density polyethylene (LLDPE) to prepare LLDPE/silica or LLDPE/Aluminun trihydrate (ATH)/silica composites. Effects of surface modification of nano-silica on the dispersibility of silica in LLDPE and combustion behaviors of composites were investigated by Transmission electron microscopy (TEM) and cone calorimeter (CONE). The results showed silica without surface modification was not well dispersed in LLDPE owing to the severe aggregation of silica particles and its poor compatibility with LLDPE. Improved dispersibility of surface modified silica was found because of its improved compatibility with LLDPE. Obvious decrease of peak-HRR value was observed. Well dispersed silica after surface modification, as proved by TEM, should be contributed to the decreased peak-HRR. Because the shapes of two curves were similar, it was concluded that the mechanistic reason for the decreased HRR come mainly from the physical effects of silica on the surface.