Carbon dioxide is a promising alternative to conventional working fluids in convection-based energy systems due to its environmentally benign nature and outstanding thermophysical properties under supercritical pressure conditions. Here, the flow and heat transfer characteristics of supercritical CO2 are numerically studied in a natural circulation loop which is a promising option in many applications such as nuclear reactors, solar heaters, electronic cooling systems, geothermal applications, etc. It is found that the two-dimensional effect of the convection motion is significant, and the special property of CO2 is favorable for the motion and heat transfer. Results indicate that the convection motion is accelerated and the heat transfer performance is greatly improved by setting heat sink temperature slightly lower than pseudo-critical temperature. Loop inclination also has significant effects on heat transfer performance. It is hoped that the present results are useful for both the efficient utilization of CO2 in engineering and the optimal operation of convection-based energy systems.