Three-dimensional　simulations　are　performed　numerically　to　investigate　the　influence　of　inlet/outlet　positions　(Z-,　I-,　and　C-type)　and　header　forms　(rectangular,　symmetric　trapezoidal,　and　triangular)　on　the　flow　and　thermal　performance　in　microchannel　heat　sinks.　The　conjugated　heat　transfer　model　is　solved　based　on　finite　element　method.　A　good　agreement　is　found　between　the　numerical　results　and　theoretical　data.　A　detailed　analysis　of　flow　and　pressure　distributions　is　proposed　to　explain　the　flow　or　heat　transfer　performance　of　different　geometric　structures.　Results　indicate　that　the　performance　of　I-type　with　a　rectangular　header　in　terms　of　flow　velocity　uniformity　and　heat　transfer　is　better　than　others　under　the　same　pumping　power　consumption.