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 Gr(b)/Re-b(2.0) (rho(b)/rho(w)) x/d criterion found to give the best prediction accuracy for both types of tubes. The threshold for die 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. (C) 2018 Elsevier Ltd. All rights reserved.