A heat radiation equation contains 4th exponentialorder of temperature which makes mathematics analysisvery complicated, therefore, most heat transfer expertsand scholars believe, based on their own experiences, thatthe heat radiation effect can be ignored due to smalltemperature difference between duct surfaces andsurrounding to simplify analysis. Due to this reason,examples shown in most heat transfer, air conditioningand refrigeration text books, and commercial design ofheat exchanger, commonly ignore the effect of heatradiation. This paper studies in detail for complete heattransfer characteristics of a circular duct with heatradiation effect taken into account. It is found that, insome practical conditions, the heat radiation effect cannot be ignored especially in cases of lower ambientconvection heat coefficients and larger surfaceemissivities, as well as the smaller the duct size or thegreater the duct conductivity or internal fluid convectioncoefficients, even though the temperature-differencebetween inner and outer fluid-temperatures of duct isvery low (such as 1 ). In most situations, ℃ ignoring heatradiation is likely to produce large errors and degrade thedesign quality of a heat exchanger. It is also proved that alarger surface emissivity can greatly improve theperformance of a heat exchanger.