The internal convective heat transfer characteristics of n-decane at supercritical pressure in a vertical square tube with a hydraulic diameter of 1.8 mm were experimentally investigated. The external wall temperatures of the square tube were measured with welded thermocouples whereas the internal wall temperatures were calculated with space marching method. In this experiment, the operating pressure ranged from 3 to 5 MPa and the heat flux from 100 to 500 kW/m(2). Besides, the flow direction in the vertical tube was also switched in order to examine the effects of buoyancy. Moreover, the circumferential uniformity of heat transfer was also discussed. The results showed that the effects of buoyancy on the heat transfer were negligible once the local buoyancy number was less than a threshold. After the threshold, the buoyancy promoted the heat transfer in the downward flow configuration, but weakened the heat transfer in the upward flow one. The reason could be inferred that the buoyancy would influence the intensity of turbulent kinetic energy and the heat transfer near the wall. Finally, new heat transfer correlations for vertical upward and downward flow in square tubes were proposed. (C) 2019 Elsevier Ltd. All rights reserved.