The effective use of wall radiation can enhance heat transfer of high-temperature heat exchangers for Stirling engines. A heat exchanger consisting of flattened U-tubes with a staggered arrangement is equipped with radiation plates and radiating porous media. Both the radiation plates inserted between the tubes and the porous media installed downstream of the tube bundle are heated by high-temperature combustion gas through convection, , and these heat the tubes by wall radiation, working as a kind of extended surface. The radiation effect is predicted to enhance the heat transfer coefficients at the outer tube surface by about 50%, which depends on the emissivities and the combustion gas temperatures. This heat transfer performance is also compared with that of a round U-tube bundle.