The　heat　transfer　enhancement　of　microchannel　heat　sinks　with　periodic　expansion-constriction　cross-sections　is　investigated　both　experimentally　and　numerically.　Each　heat　sink　consists　of　10　parallel　microchannels　with　0.1　mm　wide　and　0.2　mm　deep　in　constant　cross-section　segment　and　each　microchannel　consists　of　an　array　of　periodic　expansion-constriction　cross-sections.　Three-dimensional　laminar　numerical　simulations,　based　on　the　Navier-Stokes　equations　and　energy　equation,　are　obtained　for　pressure　drop　and　heat　transfer　in　these　microchannel　heat　sinks　under　the　same　experimental　conditions.　Multi-channel　effect,　entrance　effect,　conjugate　heat　transfer,　viscous　heating　and　temperature　dependent　properties　are　considered.　It　is　found　that　the　numerical　predictions　of　apparent　friction　factor　and　Nusselt　number　are　in　good　agreement　with　experimental　data.　The　influences　of　periodic　expansion-constriction　cross　sections　on　pressure　drop,　heat　transfer　and　thermal　resistance　are　discussed,　respectively.　The　effects　of　the　entrance　and　exit　plenum　regions　and　the　lateral　parts　of　silicon　wafer　on　fluid　flow　and　heat　transfer　are　discussed.　Special　attentions　are　given　to　analyze　the　variation　of　thermal　resistance　for　each　term　with　pumping　power,　corresponding　to　three　stages　of　heat　release　at　the　substrate　of　heat　sink.　(C)　2013　Elsevier　Ltd.　All　rights　reserved.