Heat transfer to supercritical CO2 in a horizontal pipe is investigated using direct numerical simulation (DNS). A well resolved DNS eliminates the uncertainty brought by turbulence modeling. The small pipe diameter (D = 1 mm, 2 mm) with a moderately low inlet Reynolds number (Re-0 = 5400) can be compared to the channel flow in a compact heat exchanger, e.g. a printed circuit heat exchanger (PCHE). In our simulation, the inflow temperature To is set to be lower than the pseudo-critical temperature T-pc. The thermo-physical properties change rapidly when the fluid temperature rises across T-pc under heating conditions. In the present DNS, the wall temperature T-w is found to be variable in the circumferential direction. The magnitude of T-w is higher at top than at the bottom surface. As a result of buoyancy, flow stratification with low density in the upper region of pipe is developed. The streamwise velocity field (U) over tilde (z), is also modified by the flow stratification. Low-velocity flow near the circumferential wall is heated firstly and transported to the top region by the secondary flow. High-velocity bulk fluid is concentrated at the bottom as a result of high density. It is also observed that the turbulent kinetic energy and the radial turbulent heat flux are strongly suppressed near the top surface. The attenuated momentum transport and heat transfer enhance the flow stratification. A further analysis shows a significantly decreased turbulence production in this position. (C) 2016 Elsevier B.V. All rights reserved.