In　the　present　work,　multi-walled　carbon　nanotubes　(MWCNTs)　are　used　as　additives　for　enhancing　the　thermal　conductivity　of　the　widely-used　silicon　grease　as　thermal　interface　materials　(TIMs).　The　effects　of　the　MWCNTs　length　and　surface　treatment　on　the　silicon　grease　conductivity　are　studied.　The　results　show　that　the　MWCNT　length　has　strong　influences　on　the　thermal　performance　of　the　grease　with　MWCNT　additives.　The　equivalent　thermal　conductivity　of　the　composite　layer　of　the　grease　and　the　solid　surfaces　increases　as　the　MWCNT　length　decreases.　Only　the　length　of　MWCNTs　is　short　enough,　their　addition　can　improve　the　thermal　performance　of　the　common　silicon　grease.　Surface　treatment　on　MWCNTs　can　improve　the　thermal　performance　of　the　grease.　Through　transmission　electron　microscope　(TEM)　observation　it　is　found　that　after　the　modification　of　the　short　MWCNTs　the　MWCNT　entanglement　phenomenon　in　the　mother　silicon　grease　is　weakened　and　the　MWCNTs　are　dispersed　more　uniformly.