Turbulent mixed convection associated with upward and downward flows in heated vertical tubes is investigated using the direct numerical simulation technique. With increasing heat flux, the skin friction first decreases and then increases in upward heated flow, while it changes little in downward heated flow. The heat transfer coefficient exhibits a similar trend in upward heated flow, but it monotonically increases in downward heated flow. The log laws of the mean-velocity and temperature profiles are valid for downward heated flow but not for upward heated flow. Finally, the influence of buoyancy on turbulent transport of momentum and heat is elucidated by the 'external' and 'structural' effects.