The performances of XLPE and EPR as materials candidate for insulation of high voltage power cables are investigated. Data obtained from laboratory tests performed on cable models aged under single thermal and electrical stresses, as well as under the combination of the two stresses, are considered. The drawing of eleci rical, thermal and multi-stress life lines provides a synthetic and exhaustive description of the behavior of the tested materials, :and allows the determination of endurance indices, useful for electrical, thermal and multi-stress characterization.