To　improve　the　efficiency　and　accuracy　of　cable　ageing　assessment,　a　new　method　based　on　residual　antioxidant　(AO)　content　is　proposed.　Cross-linked　polyethylene　(XLPE)　cable　insulation　is　thermally　aged　in　laboratory　at　110,　130　and　140°C　from　240　h　to　3600　h.　The　ageing　degree　of　XLPE　is　characterized　by　tensile　testing,　AC　breakdown　measurement,　and　Fourier　transform　infrared　spectroscopy.　The　residual　AO　content　in　XLPE　is　detected　by　FTIR　as　well　as　by　oxidation　induction　time　(OIT)　measurement.　It　is　observed　that　residual　AO　content　measured　by　FTIR　decreases　monotonously　with　ageing　time　while　OIT　shows　a　non-monotonic　transition.　The　results　demonstrate　that　thermal　deterioration　is　closely　related　to　residual　AO　content　and　a　critical　value　of　residual　AO　content　exists.　During　early　stage　of　thermal　ageing,　the　residual　AO　content　is　larger　than　the　critical　value　and　the　variation　in　XLPE　properties　is　relatively　small.　When　the　residual　AO　content　decreases　below　the　critical　value,　significant　deterioration　occurs.　The　close　relationship　between　thermal　degradation　and　residual　AO　content　was　analyzed　by　oxidative　chain　reactions.　©　1994-2012　IEEE.