Entropy generation of a radiative participating gas flow in rectangular ducts is studied. The thermodynamic efficiency for high-temperature flow is also defined. The critical thermodynamic regions for absorption, emission, and scattering processes in the rectangular ducts are identified, and the aspect ratio for minimum entropy generation is determined. The study is extended by considering the thermodynamic effects of Reynolds number, Boltzmann number, and average transverse Nusselt number. It is shown that the total entropy generation rate and the transport efficiency of duct flows can be controlled through Reynolds number, Boltzmann number, and Nusselt number.