An experimental study for convective heat transfer between a cavity wall surface and the external main stream has been made in the range of Reynolds numbers from 10 4 to 105. The temperature distribution in a cavity of which both width and depth are 25mm is almost uniform except a thin portion immediately adjacent to the wall surface of a cavity. The representative mean temperature of the fluid in a cavity is used for the following treatment. Convective heat transfer between a cavity wall surface and the external main stream is treated by the following two phenomena; one the heat transfer related to the temperature difference between the wall surface and the fluid in a cavity, and the other the heat transfer related to the temperature difference between the fluid in a cavity and the external main stream. Experimental data obtained for the foregoing dimensions almost coincide with existing data obtained for a cavity of 50mm in width. Approximate dimensionless correlations are described for the foregoing two phenomena.