An　improved　microchannel　heat　sink　with　the　sinusoidal　wavy　sidewall　(SIN)　is　designed　and　fabricated　by　MEMS　technique.　The　synchronous　optical　measurement　system　is　established.　The　flow　and　heat　transfer　performance　in　SIN　microchannels　is　experimentally　studied　using　acetone　as　the　working　fluid.　At　mass　fluxes　of　251　kg/(m(2).s)-423　kg/(m(2).s)　and　heat　fluxes　of　0　kW/m(2)-601　kW/m(2),　the　pressure　drops,　boiling　curves,　and　heat　transfer　coefficient　are　investigated　and　analyzed.　The　temperatures,　pressure　drops,　and　corresponding　flow　patterns　in　the　stable　state　are　emphatically　examined.　The　experimental　results　show　that,　a　continuously　developing　liquid　film　always　exists　near　the　sidewall,　which　not　only　timely　supplies　the　liquid　for　nucleation　tending　to　occur　in　the　corner　regions,　but　effectively　prevents　the　microchannel　dry-out.　Consequently,　the　premature　onset　of　nucleate　boiling　(ONB),　the　delayed　critical　heat　flux　(CHF),　and　about　100%　enhancement　of　heat　transfer　coefficient　can　be　achieved　with　SIN　microchannels　compared　to　the　traditional　rectangular　(R)　microchannels.　In　the　stable　state,　all　the　dominant　flow　patterns　at　different　positions　are　almost　fixed,　which　result　in　stable　heat　transfer　enhancement.　The　flow　pattern　transitions　between　single　liquid/bubbly　flow,　bubbly/slug　flow,　bubbly/annular　flow　are　observed　at　the　channel　midstream　in　the　low/medium/high　heat　flux,　respectively.　It　is　noteworthy　that　all　of　the　dominant　flow　patterns　at　the　channel　downstream　are　not　vapor　flow　but　annular　flow　with　thin　and　continuous　liquid　film.　In　addition,　the　flow　boiling　instabilities　characterized　by　the　temperatures　and　pressure　drops　with　obvious　fluctuations　are　also　illustrated.