Epoxy-based　materials　are　widely　used　in　electronic　devices　as　the　main　insulation　due　to　their　excellent　dielectric　and　thermal　properties.　However,　thermal　cycles　originating　from　the　drastic　changes　in　working　temperature　can　greatly　accelerate　their　degradation.　In　this　paper,　the　effect　of　thermal　cycle　ageing　(-55　~　150　°C)　on　the　AC　breakdown　strength　(EB)　of　neat　epoxy　(EP)　and　micron　boron-nitride/epoxy　(BN/EP)　composites　are　investigated.　The　aging　process　can　be　understood　by　two　stages.　During　the　first　stage,　EB　of　EP　experienced　an　evident　rise　from　64.73　to　76.17　kV/mm,　and　that　of　BN/EP　displayed　a　similar　growth　from　73.01　to　77.41　kV/mm.　The　variation　of　their　glass　transition　temperature　(Tg)　was　consistent　with　EB.　Results　of　Fourier　transform　infrared　spectroscopy　and　the　trap　characteristics　indicate　that　post-curing　of　epoxy　matrix　and　the　effect　of　low　temperature　were　both　responsible　for　the　initial　increase.　In　the　later　stage,　owing　to　the　effects　of　thermal-oxidative　reactions　of　epoxy　resin,　EB　of　EP　dropped　to　63.11　kV/mm　at　150　cycles.　There　was　a　more　drastic　decline　of　BN/EP　to　61.38　kV/mm,　and　this　severer　deterioration　might　be　attributed　to　the　thermal　expansion　coefficients　mismatch　between　BN　fillers　and　epoxy　matrix.　©　2021　IEEE.