This numerical study investigates the heat transfer, pressure drop, and flow characteristics for buoyancy-assisting and opposing flow of water in the simultaneously hydrodynamically and thermally developing laminar-turbulent transitional regime of mixed convection in a vertical tube. Two-dimensional axisymmetric steady-state simulations were carried out for Reynolds number (Re) from 2000 to 5000, Grashof number (Gr) from 4×10⁵ to 2.5×10⁶, and Richardson number (Ri) 0.1 with a length-to-diameter (L/D) ratio of 150, subjected to constant heat flux boundary condition from the tube walls. Numerical simulations were performed using the pressure-velocity coupling scheme and second-order UPWIND scheme for momentum, energy, and other transport quantities. Two transition models have been compared, and the model with the better performance, known as the Transition Shear Stress Transport (SST), is used for the simulations. Results show that buoyancy plays a significant role in laminar-turbulent transition in assisting and opposing flows. The pressure drop and heat transfer increases with the increase in Re at fixed Ri in both the flows. In addition to it, at a given Ri, the pressure drop as well as heat transfer both are higher in opposing flow. The effect of heat flux on the entry length is also analyzed in buoyancy-assisting and opposing flow. It decreases in both the flows with the increase of heat flux. Furthermore, the start of transition depends on the heat flux supplied and it gets delayed with the increase of it.