Effects of combined buoyancy forces of thermal and mass diffusion on turbulent forced convection heat and mass transfer in downward flow were numerically examined in detail. Results are presented for air-water system under different conditions. Particular attention is paid to investigating the role of latent heat transport associated with the film vaporization in turbulent mixed convection heat and mass transfer. Predicted results show that the heat transfer along the gas-liquid interface is dominated by the latent heat transfer. The results also show that the assumption of an extremely thin film thickness made by Lin et al. [J. Heat Transfer 110, 337-344 (1988)] is inappropriate, particularly for a system with a large liquid mass flow rate B. Additionally, it is found that the opposing-buoyancy forces would cause an enhancement in heat and mass transfer results compared with the corresponding results of forced convection. And the extent of enhancement in heat and mass transfer coefficients increases with the increase in T-w or q"(w).