The annular thin film condensation of water vapor in wet flue gas flowing through a vertical tube was studied theoretically and experimentally. Especially discussions were conducted on the effects induced by condensation of small amount of water vapor (10 = 20% fraction) on convection heat transfer in a vertical tube. The convection heat transfer was enhanced by the condensation of the condensable gas (water vapor) existing in the wet flue gas. The experiments also indicate that the wall temperature was an important factor affecting the condensing rate and the fog formation in the wet Rue gas. The fog formation in the wet flue gas has a significant influence on the condensing rate and associated heat transfer. The proportion of the sensible and latent heat contribution to total heat transfer would change with the Re number. When the wall temperature is much lower than the saturation temperature of the inlet wet flue gas, the effect of the superheat could be neglected. But when the fog forms in the bulk flow, the temperature and concentration profile would be altered.