The　condensation　performance　of　R134a　in　horizontal　round　tubes　and　flattened　tubes　was　numerically　investigated.　Three　round　tubes　with　different　hydraulic　diameters　(d(h)　=　3.25,　4.57　and　6.5　mm)　were　deformed　into　flattened　tubes　with　the　aspect　ratio　(AR)　equals　to　2,　4　and　6,　respectively.　The　liquid　film　thickness　combined　with　the　interface　line　was　shown　in　detail　to　illustrate　the　effect　of　shear　stress,　surface　tension　and　gravity.　The　heat　transfer　coefficients　increased　with　the　vapor　quality　and　aspect　ratio,　and　the　enhancement　was　magnified　at　higher　vapor　quality　and　mass　velocity.　The　smaller　the　original　round　tube　diameter　was,　the　higher　the　enhancement　of　thermal　performance　in　flattened　tubes　was.　The　average　film　thickness　of　flattened　tubes　with　AR　=　2,　4,　6　was　respectively　35%,　50%　and　60%　thinner　than　that　of　round　tube.　The　relative　role　of　surface　tension　and　gravity　determined　the　local　thermal　performance　in　flattened　tubes.　The　main　thermal　resistance　was　located　in　the　arc.　The　gravity　effect　was　almost　negligible　in　the　straight　part　but　still　played　an　important　role　in　the　arc　of　flattened　tubes.　The　heat　exchanging　process　mainly　occurred　near　the　intersection　of　straight　part　and　arc　and　this　location　tends　to　move　toward　the　straight　side　for　smaller　flattened　tubes　because　of　the　reinforcement　of　the　surface　tension　effect.　The　pressure　gradient　also　increased　with　the　vapor　quality,　aspect　ratio　and　mass　velocity　but　decreased　with　the　increasing　hydraulic　diameter.　The　existing　correlations　almost　tend　to　underestimate　both　the　heat　transfer　coefficients　and　pressure　gradients.　Two　improved　correlations　of　condensation　heat　transfer　and　pressure　gradient　in　flattened　tube　were　developed　for　practical　applications.　(C)　2017　Elsevier　Ltd　and　IIR.　All　rights　reserved.