Abstract: For the purpose of analyzing the effects of wall sub-cooling on the heat transfer characteristics of forced convection condensation for steam/air mixtures in a horizontal tube, the experiments under air-cooling and water-cooling have been conducted. By comparing the experimental data for different inlet air mass fractions, mixtures velocities and coolant volume flow rates, the variation of local heat transfer coefficient with wall sub-cooling was obtained. The results show that for annular and wavy flow, the condensation heat transfer coefficient increases with wall sub-cooling while decreases with wall sub-cooling for stratified flow. The positive effects of the increasing wall sub-cooling on the condensation heat transfer coefficient is enhanced with the rise of inlet air mass fraction and mixture gases velocity for annular and wavy flow.