The equations for condensation in cooled laminar tube flows are presented. The solutions for low vapor concentrations with variable vapor-gas thermodynamic properties are obtained. The problem, which is coupled with the aerosol size distribution, is treated by the particle source in cell (PSI-CELL) method. The effect of a mixed convection regime, where particle trajectories are not straightforward and their paths depend on their sizes and local points, is considered. The geometric standard deviation or particle size distributions and the mass fraction of particles are compared in the cases of an upward and a downward mixed convection region. Wall loss in the upward How is found to be smaller than that in the downward flow. The monodispersity of particle size distributions in the downward flow is higher than that in the upward flow. As the tube diameter increases, aerosol mass fraction and particle size increase and the particle number density decreases.