The　skin-core　structure　of　thermally　aged　epoxy　resin　consisting　of　the　aged　surface-layer　and　the　unaged　inner-layer,　was　studied　in　this　paper.　A　new　tanδ　peak　appeared　in　the　results　of　dynamic　mechanical　analysis　(DMA)　after　the　specimens　were　thermally　aged.　It　steadily　diminished　when　the　surface　of　the　specimens　was　gradually　removed,　which　indicates　that　the　newly　detected　peak　arose　from　formation　of　surface-layer　during　ageing.　Thickness　of　the　surface-layer　could　reach　up　to　0.09　mm　after　thermal　ageing　for　480　h.　According　to　Fourier　transform　infrared　spectroscopy　(FTIR)　results,　formation　of　the　surface-layer　was　found　to　closely　correlated　with　thermal　oxidation　and　re-crosslinking　processes.　The　former　induced　oxidized　molecular　chains　while　the　later　might　be　related　to　the　production　of　carboxylic　acid　dimers　and　etherification　reactions　to　result　in　strengthened　networks.　This　was　further　ascertained　by　the　results　of　dielectric　spectroscopy　and　isothermal　surface　potential　decay　(ISPD)　tests.　The　relaxation　process　of　aged　epoxy　resin　displayed　higher　relaxation　time,　strength,　and　activation　energy　than　unaged　specimens,　and　evident　growths　of　both　deep　and　shallow　level　traps　densities　were　also　observed　after　thermal　ageing.　This　casts　new　light　on　the　thermal　ageing　mechanism　of　epoxy　resin　and　could　provide　a　feasible　method　for　its　insulation　degradation　assessment.　©　2021　Elsevier　Ltd