System stabilities are of critical importance in natural circulation applications. Investigations show that for supercritical CO2 flow new behaviors can be seen in natural convection systems considering its temperature-sensitive physical properties. In the present study, numerical simulations on a supercritical CO2 natural circulation loop have been carried out to investigate the flow transitions and instabilities of such systems. In the present model heat sink temperature is kept at 298 K while heat source temperature varies in the range of 310-1023 K as a controlling parameter. It is found for the first time that for the present supercritical CO2 model there exists a transition heat source temperature at which the system changes from unstable repetitive-reversal flow into stable one-direction flow with the increase of temperature, which is fundamentally different from previous studies for normal fluid. In particular, the critical transition fluid temperature is found to be near the second "pseudo-critical temperature" at around 375 K where the fluid properties experience major transitions with the increase of temperature. In addition, characteristics of flow stability behaviors are also analyzed in detail. (C) 2010 Elsevier Ltd. All rights reserved.