The heat transfer of natural convection from the horizontal heated surface which a heated annular duct was placed above was investigated experimentally. The relationship of the flow and heat transfer characteristics to the height and the heat flux of the heated ducts were examined. With decreasing the height, the induced flow rate decreases and the reverse flow occurs. The heat transfer increases with increasing the modified Rayleigh number of the heated ducts. However, when flow reversal reaches to the horizontal heater, the heat transfer has a little influence on that.