Polyethylene (PE) provides good flexibility at low temperatures, chemical stability, and electrical performance that makes it an ideal insulating material for wire and cable, but it has low melting point and is flammable. In fire events, PE must be modified with additives to enhance its flame retardancy and satisfy the fire safety standards. In this study, we used the LDPE and cross-linked polyethylene (XLPE) as the matrix, added nanoclay montmorillonite (MMT), flame retardant magnesium hydroxide (MH) and compatibilizer LDPE grafted maleic anhydride (LDPE-g-MA). The PE/PE-g-MA/MH/MMT nanocompo-sites were compounded using the twin screw extruder, then were injection molded into the standardized specimens. The properties of nanocomposites were characterized with XRD, TGA, and DSC techniques and LOI, tensile, and impact examinations. The results showed that the interlayer of OMT (modification of organoclay) modified with dodecylamine expanded to about 4.1 A˚ and there was no 2θ peaks between 1.5° and 10°, indicating that the dispersion of nanocomposites were exfoliation structure from XRD pattern. The tensile and impact strengths were enhanced with LDPE-g-MA and clay added. The increase in char residual of nanocomposites indicated that the thermal stability at elevated temperatures improved. The XLPE nanocomposites achieved the effective level of flame retardancy.