In　the　present　study,　fluid　flow　and　heat　transfer　in　microchannel　heat　sinks　with　different　inlet/outlet　locations　(I,　C　and　Z-type),　header　shapes　(triangular,　trapezoidal　and　rectangular)　and　microchannel　cross-section　shapes　(the　conventional　rectangular　microchannel,　the　microchannel　with　offset　fan-shaped　reentrant　cavities　and　the　microchannel　with　triangular　reentrant　cavities)　are　numerically　studied　with　computational　domain　including　the　entire　microchannel　heat　sink.　Detailed　three-dimensional　numerical　simulations　are　useful　in　identifying　the　optimal　geometric　parameters　that　provide　better　heat　transfer　and　flow　distribution　in　a　microchannel　heat　sink.　Results　highlight　that　flow　velocity　uniformity　is　comparatively　better　for　I-type　and　poor　for　Z-type.　The　flow　distribution　is　found　to　be　symmetrical　for　I-type.　It　is　seen　from　the　header　shapes　analysis　that　the　rectangular　header　shapes　provides　better　flow　velocity　uniformity　than　the　trapezoidal　and　triangular　headers.　The　fluid　flow　mechanism　can　be　attributed　to　the　interaction　of　the　branching　of　fluid　and　the　friction　offered　by　the　walls　of　the　header.　Effects　of　microchannel　cross-section　shapes　emphasize　that　the　microchannel　with　offset　fan-shaped　reentrant　cavities　and　the　microchannel　with　triangular　reentrant　cavities　of　the　heat　sinks　enhance　the　heat　transfer　compared　to　the　conventional　rectangular　microchannel.　The　heat　transfer　mechanism　can　be　attributed　to　the　jetting　and　throttling　effect,　the　additional　flow　disturbance　near　the　wall　of　the　reentrant　cavities　and　the　form　drag　of　the　reentrant　cavities.　The　heat　sink　C　has　better　heat　transfer　characteristic　for　q(v)　=　150　ml/min　and　is　able　to　prolong　the　life　of　the　microelectronic　devices.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.