The problem of the transient laminar mixed convection flow of air in a vertical tube under buoyancy effect and high wall heat flux condition has been numerically investigated by using a full 3D-transient-model and Boussinesq's assumptions. The tube is submitted to a uniform but time-dependent wall heat flux. Results have clearly shown that the flow reversal was first initiated near the tube exit section, on the tube wall and at the level Gr = 3 × 10 5 for opposed-buoyancy case, and on the tube centerline and at the level of Grashof number around 10 6 for assisted-buoyancy one. With further increase in time of the wall heat flux, such reversed flow region has considerably increased in size and intensity and it clearly spreads into the upstream region. The presence of such recirculation cells has drastically perturbed the flow and the thermal field as well as the heat transfer. Although the flow structure appears to conserve its axisymmetrical characters even for cases with very high Grashof numbers, results from this study have shown that the flow seems to remain stable and unique up to the level Gr = 5 × 10 5 and 10 6 , respectively, for opposed and assisted buoyancy cases. Beyond these critical Grashof numbers, an extremely slow and rather difficult and tedious convergence behaviors have been experienced, which are believed to be due to a possible flow transition.