The multimode heat transfer characteristics from the combination of mixed convection and radiation are studies in this research. Mixed convection heat transfer is largely influenced by the flow, surface emissivity, geometry of the duct and location of heater. In this research, the horizontal square duct is isothermal heated from different location. The aims of this research are to study the heat transfer characteristic within horizontal square duct, to construct and fabricate the square test section, to determine the effect of heater location on the overall heat transfer rate, to determine the effect of Reynolds number to heat transfer within square duct and to determine the effect of emissivity on the overall heat transfer rate. This research also provide the flow visualization for bottom, side and top heating case and determine the heating condition that results in the best heat transfer.