The　characteristic　of　fluid　flow　and　heat　transfer　in　microchannel　heat　sinks　was　investigated　theoretically　and　experimentally.　Firstly,　the　theoretical　model　of　convective　heat　transfer　in　the　microchannel　heat　sink　was　presented.　Secondly,　the　values　of　pressure　drop　and　Nusselt　number　were　measured　and　calculated　based　on　the　experiments.　Furthermore,　the　experimental　results　were　compared　with　those　obtained　from　theoretical　model,　average　error　about　10%.　Lastly,　the　conductive　thermal　resistance,　convective　thermal　resistance　and　capacitive　thermal　resistance　were　also　analyzed　under　different　Reynolds　numbers　and　aspect　ratios　of　channel.　Results　indicate　that,　the　convective　thermal　resistance　plays　an　important　role　in　the　heat　transfer　performance　of　microchannel　heat　sinks,　with　the　proportion　about　90%　when　Reynolds　number　and　aspect　ratio　of　channel　are　985　and　1,　respectively;　while　the　conductive　thermal　resistance　can　be　neglected　(less　than　10%)　under　small　Reynolds　number,　and　the　capacitive　thermal　resistance　to　total　thermal　resistance　decreases　with　the　increasing　Reynolds　number　and　aspect　ratio　of　channel.　©,　2015,　BUAA　Press.　All　right　reserved.