A numerical study was made on convective heat transfer in supercritical carbon dioxide contained in a two-dimensional duct that consisted of two parallel walls and two adiabatic walls. The temperatures of the parallel walls were set at 305 and 306K, and the pressure was varied between 7.5 MPa and 9.5 MPa. At 7.5 MPa, which is near the critical pressure of carbon dioxide, the velocity and temperature profiles were highly unsymmetric and skewed. The variation of Nusselt number with Rayleigh number for supercritical carbon dioxide was found to have a similar slope to those for atmospheric air and water systems.