The developing process of the mixed convection flow in a circular tube, the lower half of which is maintained at the constant heat-flux condition, is examined. Using a fully elliptic Navier-Stokes procedure, which is made to satisfy global mass continuity at each interior section, calculations have been performed for Pr = 0.7 and 5 at Re = 250. For Gr between 10(6) and 10(7), the vortical secondary flow develops almost immediately and increases the heat transfer considerably. The active crossflow motion can cause the flow in the upper region to reverse early in the developing region. The natural developed flow is of two-vortex type. However, as the temperature gradient builds up near the bottom surface, a new pair of vortices could be developed. The flow, in the present parameter range, is shown to develop either type of solution. The four-vortex solution is more likely to occur as Pr and/or Gr increase(s). The details of the developing process are presented.