In　this　study,　the　effect　of　Al2O3-R141b　nanofluids　as　the　working　fluid　on　the　thermal　performance　of　micro-channel　surfaces　at　different　pressures　and　nanofluid　particle　concentrations　were　experimentally　investigated.　The　operational　pressures　and　volume　concentration　of　nanoparticles　were　varied　among　0.86　x　10(5),　0.91　x　10(5),　0.96　x　10(5),　1.0　x　10(5),　and　2.0　x　10(5)　Pa,　and　0.001,　0.01,　and　0.1　vol%,　respectively.　The　experimental　results　showed　that　the　operational　pressure　and　volume　concentration　of　nanoparticles　have　significant　effect　on　the　heat　transfer　characteristics　for　nanofluids　of　Al2O3/R141b.　For　operating　pressure　in　the　range　0.86-0.96　x　10(5)　Pa　and　nanofluid　concentrations　of　0.001　and　0.01　vol%,　the　corresponding　heat　transfer　coefficients　were　larger　than　those　of　the　base　fluid.　For　the　concentration　of　0.1　vol%,　when　the　superheat　degree　was　higher　than　that　for　the　pure　fluid,　the　heat　transfer　coefficient　was　lower.　At　high　pressures　of　1.0　and　2.0　x　10(5)　Pa,　for　the　concentration　of　0.1　vol%,　heat　transfer　deteriorated　for　the　entire　range　of　measured　data.　Under　the　same　operating　pressures,　for　the　concentrations　of　0.001　and　0.01　vol%,　the　heat　transfer　enhancement　was　not　apparent.　Further,　the　heat　transfer　characteristics　of　the　nanofluids　for　Al2O3-nanoparticle-covered　surfaces　with　rectangular　microchannels,　which　were　produced　by　the　evaporation　of　nanofluids　with　various　concentrations　ranging　from　0.001　to　0.1　vol%,　were　experimentally　studied.　The　heat　transfer　curves　for　the　nanoparticle-covered　surfaces　were　compared　with　those　for　nanofluids　on　bare　surfaces,　and　the　results　demonstrated　that　the　nanoparticle　covering　has　a　strong　influence　on　the　heat　transfer　behavior.　(C)　2013　Elsevier　Ltd.　All　rights　reserved.