Fluid dynamics and heat transfer of supercritical CO2 natural convection are important for nuclear engineering and new energy system design etc. In this paper, in order to study the flow and heat transfer behavior of supercritical CO2 natural circulation system, a computational simulation on a closed natural circulation loop (NCL) model has been carried out. The fluid temperature in the loop varies between 298.15 K and 323.15 K, which is across the CO2 critical temperature, and the density is found to be in the range of 250-800 kg/m(3). The results show a small temperature difference of 25 degrees C between heating and cooling sources can induce a mass flow with the Reynolds number up to 6 x 10(4) using supercritical CO2 fluid. A periodic reversal flow pattern is found and presented in this paper. Enhanced heat transfer phenomenon is also found for the supercritical CO2 natural convective flow. The mechanisms to this enhancement and the heating effect on the flow are also discussed in detail in the present study. (C) 2010 Elsevier Ltd. All rights reserved.