Cross-linked　polyethylene　(XLPE)　is　widely　used　as　insulating　material　of　high　voltage　power　cables　due　to　its　excellent　physicochemical　and　electrical　properties.　It　is　necessary　to　understand　the　thermal　ageing　of　XLPE　in　order　to　guarantee　the　reliability　of　power　cables.　In　this　paper,　the　XLPE　samples　were　subjected　to　thermal　ageing　at　155°C　in　air.　The　influence　of　thermal　ageing　on　optical　properties　of　XLPE　and　its　structural　origin　were　investigated　by　Ultraviolet-visible　spectrophotometer　(UV-Vis)　and　Fourier　transform　infrared　spectroscopy　(FTIR).　It　was　observed　that　the　color　of　XLPE　samples　changed　from　transparent　white　to　yellow　even　red　with　ageing　time.　The　UV-Vis　absorption　spectra　showed　that　the　absorption　edge　shifted　from　ultraviolet　to　visible　region　which　was　consistent　with　the　color　change.　The　optical　band　gap　Eg　calculated　from　UV-Vis　curves　was　found　to　decrease　after　thermal　ageing.　The　modification　in　optical　properties　of　XLPE　was　attributed　to　the　formation　of　conjugated　clusters　according　to　UV-Vis　and　FTIR　results.　FTIR　spectra　showed　that　the　unsaturation　and　carbonyl　groups　increased　with　ageing　time　which　contributes　to　the　formation　of　conjugated　cluster.　The　structural　changes　indicated　the　chain　scission　and　oxidation　of　XLPE　molecular　chains　by　thermal　ageing.　These　experimental　results　also　provide　a　promising　method　for　condition　assessment　of　XLPE　cable　insulation　based　on　optical　properties　of　XLPE.　©　2020　Institute　of　Electrical　Engineers　of　Japan.　All　rights　reserved.