The turbulent heat transfer of combined forced and natural convection along a vertical flat plate is investigated experimentally both on the aiding and the opposing flows of air. Local heat transfer coefficients are measured in the vertical direction. The results show that the local Nusselt numbers for the aiding flow become smaller than those for the forced and the natural convection, while the Nusselt numbers for the opposing flow are increased significantly. These results are compared with the previous results for water. It was found that the non-dimensional parameter Z (=Gr*_x/Nu_xRe_x)^2.7Pr^0.6) can predict the behavior of heat transfer both for air and water. Furthermore, the natural, the forced, and their combined convection regions can be classified in terms of the above parameter.