Numerical investigation has been performed in order to determine the onset of flow reversal occurrence for buoyancy assisted flow of supercritical CO2 through vertical annular channel of finite length with heated inner wall. The outer wall was considered adiabatic. Only steady state and laminar mixed convection cases were considered in order to study the involved heat transfer phenomena in a fundamental level. Results gave a clear insight of the flow reversal mechanism and highlighted buoyancy as the driving force of the aforementioned phenomenon. Furthermore, according to the results, two different flow reversal maps were constructed with the use of dimensionless parameters. Results clarifies that the dominance of free convection is vital for the onset of the flow reversal. More specifically, free convection must be higher from both conductive and forced convective heat transfer. In addition, mass flux suppresses the mentioned phenomenon, while heat flux aids the presence of flow reversal.