In　this　paper,　direct　current　(DC)　surface　flashover　characteristics　of　modified　polyimide　(PI)　are　studied　in　vacuum　(10-4Pa).　Two　kinds　of　semi-conductive　powders　(nano　scale,　micro　scale)　are　used　as　modifier　of　raw　PI.　Meanwhile,　the　surfaces　of　flashover　experimented　samples　are　observed　using　scanning　electron　microscopy　(SEM),　and　the　influence　of　the　secondary　electron　emission　(SEE)　coefficient　on　DC　surface　flashover　characteristics　is　discussed.　Experimental　results　show　that,　(1)　In　vacuum,　DC　surface　flashover　voltage　of　the　nano　modified　PI　is　higher　than　pure　PI,　however　that　of　micro　modified　PI　is　lower　than　pure　PI;　(2)　The　surface　SEE　coefficient　of　nano　modified　PI　is　smaller　than　pure　PI.(3)　Gas　overflow　phenomenon　appears　in　the　moment　of　prebreakdown.　In　combination　with　the　results　of　SEM,　we　infer　that　the　gas　desorption　is　the　key　factor　which　affects　the　DC　surface　flashover　voltage　in　vacuum.　©　2012　IEEE.