Experimental　results　of　flow　boiling　characteristics　and　flow　patterns　with　acetone　in　two　different　microchannel　heat　sinks　are　presented　in　this　paper.　A　semi-open　microchannel　heat　sink　and　a　straight　microchannel　heat　sink　with　19　parallel　microchannels　each　were　designed　and　tested.　The　semi-open　microchannels　have　a　channel　width　of　0.8　mm,　fin　width　of　0.4　mm,　and　pedestal　height　of　0.2　mm　and　the　straight　microchannels　have　a　rectangular　cross　section　of　0.8mm　x　1mm.　The　experimental　heat　fluxes　ranged　from　0　to　90kW/m(2),　vapor　quality　ranged　from　0.05　to　0.5,　mass　fluxes　ranged　from　4.34　to　15.62kg/m(2)s　and　the　inlet　temperatures　were　20　and　30　degrees　C,　respectively.　Compared　to　those　in　the　straight　microchannels,　flow　boiling　heat　transfer　coefficients　can　be　improved　by　up　to　36.2%.　Furthermore,　flow　patterns　were　observed　with　a　speed　video　camera.　The　flow　boiling　heat　transfer　mechanisms　are　analyzed　according　to　the　observed　flow　patterns.