This experimental study concerns the mixed, forced and natural convection heat transfer in rectangular cross-section cavities heated from bottom surface. Flow visualization, temperature profiles and measurements of heat transfer from bottom heating surface are performed for various free-stream velocities, temperature differences between the free-stream and the heating surface and dimensions of the cavity. It is found that flow pattern and amount of convective heat transfer are influenced by an interaction between forced and natural convection, and there is an existence of Reynolds number's region which the amount of convective heat transfer is decreased with increasing Reynolds number. This paper correlates the experimental data in terms of Nusselt number, dimension ratio of cavity (depth/width of cavity), Grashof and Reynolds numbers according to the region of Reynolds numbers.