Measured　heat　transfer　rates　of　supercritical　R134a　were　compared　for　flows　in　a　horizontal　micro-fin　tube　and　in　a　smooth　tube　for　mass　fluxes　from　100　to　700　kg　m−2　s−1,　heat　fluxes　from　10　to　70　kW　m−2,　and　pressures　from　4.26　to　5　MPa.　The　results　showed　that　the　micro-fin　tube　wall　temperatures　had　smaller　changes　as　the　heat　flux　was　increased　than　on　the　smooth　tube.　The　heat　transfer　coefficients　along　the　top　of　the　smooth　tube　are　reduced　more　by　the　buoyancy　effect　than　in　the　micro-fin　tube.　Then,　the　results　were　used　to　evaluate　four　buoyancy　criteria　for　horizontal　flows　for　both　the　smooth　tube　and　micro-fin　tube　with　the　%[Formula　presented][Formula　presented]　criterion　found　to　give　the　best　prediction　accuracy　for　both　types　of　tubes.　The　threshold　for　the　onset　of　the　buoyancy　effect　in　the　smooth　tube　was　around　100　but　was　around　2000　in　the　micro-fin　tube,　which　indicates　that　the　effect　of　buoyancy　is　greatly　reduced　in　the　micro-fin　tube.　Comparisons　of　the　measured　heat　transfer　coefficients　for　all　5520　experimental　data　points　showed　that　the　heat　transfer　coefficient　in　the　micro-fin　tube　was　1.68　times　that　in　the　smooth　tube　on　the　top　and　1.59　times　that　in　the　smooth　tube　on　the　bottom.　©　2018