Supercritical organic Rankine cycles (ORCs) are a promising waste heat recovery technology because of their high thermal efficiency and lower exergy loss. Heat transfer characteristics must be considered for vapor generator design in ORCs. This paper focuses on the buoyancy effect on heat transfer characteristics in horizontal tubes of supercritical R134a, a widely used ORC working fluid, whose supercritical heat transfer characteristics have been less widely studied. Experiments of supercritical R134a flowing in horizontal tubes with different inner diameters of 10.3 mm and 16 mm were conducted to obtain basic experimental data. The influences of heat flux, mass flux, and tube diameter on heat transfer were investigated with an emphasis on the buoyancy effect. This study attempts to extend the applicability of existing buoyancy criteria to organic fluids in horizontal tubes to quantitatively analyze the buoyancy effect. Existing buoyancy criteria, which were developed for water, were evaluated and the threshold values determining the onset of the buoyancy effect in heated horizontal tubes were determined for the present data. A simple parameter of qd (0.7)/G(12) not based on wall temperature was developed to be correlated with the maximum temperature difference between the top and bottom surfaces for convenience in engineering use. This parameter was validated with both the present data and supercritical water data from the literature. (C) 2018 Elsevier Ltd. All rights reserved.