The problem of low Reynolds number forced convection of supercritical CO2 in a horizontal circular tube is studied numerically. The interest has been simulated by an experimental study on solar collector using supercritical CO2 as working fluid, in which high collector efficiency above 70% has been found for the supercritical CO2-based collector. In the present study, to reproduce the general features exhibited in the experiments, a circular tube having a diameter of 6.0 mm has been investigated at a pressure of 8.0 MPa and an inlet temperature of 32.0 degrees C. In particular, this work has been focused upon the convective heat transfer characteristics of CO2 flowing in a horizontal circular tube with small mass flow rates ranging from 0.003 to 0.03 kg/min and constant heat flux from 100.0 to 800.0 W/m(2). The results show that a threefold increase in Nusselt number has been achieved at the typical Reynolds number of 210 in the experimental tests, compared with that of using water as working fluid in the collector. The heat transfer enhancement is also found to increase with Reynolds number Re and heat flux q for ranges of 2 10 <= Re <= 1800 and 100 W/m(2) <= q <= 800 W/m(2). Furthen-nore, the mechanisms that are responsible for the heat transfer enhancement are identified. The first is due to thinner thermal boundary layer than that of the water case and the second is that the decrease in viscosity and the increase in heat capacity contribute to the heat transfer enhancement. In addition, the flow does not reach a fully developed velocity and temperature field, which may also contribute to the enhancement phenomena. (C) 2006 Elsevier B.V. All rights reserved.