A cross-linked polyethylene composition for a power cable insulator including (A) 100 parts by weight of a polyethylene base resin, (B) 0.1 to 0.6 parts by weight of a hindered phenol-based antioxidant, (C) 1 to 4 parts by weight of a crosslinking agent, (D) 0.2 to 1.0 parts by weight of magnesium oxide and (E) 0.1 to 1.0 parts by weight of a scorch inhibitor, which advantageously exhibits superior resistance to water tree generated when a power cable insulator is exposed to outside water and electric field, and superior electrical insulation characteristics.