Magnesium hydroxide (MH) has been identified as an alternative flame retardant for polyethylenes (PEs), to replace halogen-based flame retardants, which cause severe environmental problems. However, the use of MH in fabricating composites with PEs is limited because a high loading ratio is required for adequate flame suppression and because the hydrophilic surfaces of microparticulate MH cause aggregation in hydrophobic, highly crystalline PEs. To overcome these issues, we modified the surfaces of MH nanoparticles using hexylphosphoric acid to obtain hydrophobic surfaces. The surface-modified MH nanoparticles enhanced the thermal stabilities of PE composites, even at a low loading ratio of below 30 wt%. It was attributed to the uniform dispersion of magnesium hydroxide nanoparticles by increased association between the hexyl moieties and short alkyl side chains in polyethylenes together with the synergetic effect of the endothermic decomposition reaction of magnesium hydroxide and radical capture by phosphorus moieties.