A　numerical　investigation　has　been　carried　out　to　examine　the　characteristics　of　laminar　flow　and　heat　transfer　in　microchannel　heat　sink　with　offset　ribs　on　sidewalls.　The　three-dimensional　equations　considering　entrance　effect,　conjugate　heat　transfer,　viscous　heating　and　temperature-dependent　properties　are　solved　for　the　fluid　flow　and　heat　transfer　in　the　microchannel　heat　sink.　Five　different　shapes　of　offset　ribs　are　designed,　including　rectangular,　backward　triangular,　isosceles　triangular,　forward　triangular　and　semicircular.　Results　show　that　the　offset　ribs　result　in　significant　heat　transfer　enhancement　and　higher　pressure　drop.　Depending　on　the　different　offset　ribs　and　Reynolds　number　(190　Re　838)　studied　in　the　present　work,　Nusselt　number　and　friction　factor　for　the　microchannel　heat　sink　with　offset　ribs　are　1.42-1.95　and　1.93-4.57　times　higher　than　those　for　the　smooth　one,　leading　to　performance　evaluation　criteria　of　1.02-1.48.　Further,　as　a　consequence　of　significant　pressure　drop,　the　microchannel　heat　sink　with　offset　ribs　gradually　loses　its　advantage　as　an　effective　heat　transfer　enhancement　method　at　higher　Reynolds　number.　Crown　Copyright　(C)　2015　Published　by　Elsevier　Ltd.　All　rights　reserved.