Experimental investigations on the aerodynamic structure of a laminar boundary layer diffusion flame over a horizontal flat plate have been performed. When a fuel (methane) is injected upward, the velocity around a flame zone is locally accelerated, and the velocity gradient at the wall is increased as compared with that of an isothermal flow. On the other hand, when a fuel is injected downward, the velocity is retarded, and the velocity gradient at the wall is decreased not only in the downstream region of the leading flame edge but also in the upstream region. This shows that the gravitational force varies the local pressure distribution not only in the downstream region of the leading flame edge but also in the upstream region. It is also elucidated that the velocity gradient of the free stream velocity varies the velocity profiles in a boundary layer.