This paper presents experimental investigations of the convection heat transfer of carbon dioxide at supercritical pressures in a vertical tube with inner diameter of 99.2 mu m for various Reynolds numbers, heat fluxes and flow directions. The effects of buoyancy and flow acceleration due to heating and pressure drop are evaluated and analysed. The results show that the effects of flow acceleration are significant and the local wall temperature varies non-linearly for both upward and downward flows at the pressures in the vicinity of critical point and low inlet Reynolds numbers when the heat fluxes are relatively high. The buoyancy effect on the heat transfer is negligible in micron scale tubes at inlet Reynolds (from 2600 to 6700) and various heat fluxes (from 85 kW/m(2) to 748 kW/m(2)). The flow acceleration due to heating and pressure drop can strongly influence the turbulence and reduce the heat transfer for high heat fluxes and low inlet Reynolds. Comparison of numerical predictions with the experimental data showed that the AKN low Reynolds number turbulence model gave better agreement than the k-epsilon realizable turbulence model with the enhanced wall treatment. (C) 2012 Elsevier Ltd. All rights reserved.