This　study　aims　to　research　on　the　flow　and　heat　transfer　characteristics　of　a　flat　aluminum　extruded　multiport　tube　using　water　as　working　fluid.　This　kind　of　tube　is　composed　of　numerous　parallel　of　rectangular　microchannels　and　is　widely　used　in　industry　as　heat　transfer　unit　of　heat　exchanger.　Six　samples　with　hydraulic　diameter　ranging　from　0.48　to　0.84　mm,　aspect　ratio　varying　from　0.45　to　0.88　and　relative　roughness　ranging　from　0.29%　to　1.06%　are　carried　on　experimental　investigation　to　test　friction　factor　and　Nusselt　number.　The　test　is　performed　with　Reynolds　number　varied　from　120　to　3750,　covering　the　laminar,　critical,　and　early　transition　zone.　The　experimental　results　show　that　the　laminar　turbulent　transition　occurs　at　Re-cr　=　1200-1600　respectively　for　six　samples.　For　friction　factor,　entrance　effect　has　a　significant　impact　in　turbulent　region,　while　roughness　seems　to　be　negligible　and　no　evidence　can　be　found　to　indicate　that　aspect　ratio　has　correlation　with　the　early　transition.　For　Nusselt　number　(convective　heat　transfer　process),　both　roughness　and　entrance　effect　can　enhance　heat　transfer.　Roughness　is　more　significantly　effective　at　high　Reynolds　numbers,　while　the　entrance　effect　is　more　effective　in　the　laminar　region.　(C)　2014　Elsevier　Ltd.　All　rights　reserved.