Analysis method for the radiation heat transfer from the duct burner flame to the heat exchanger in a Heat Recovery Steam Generator (HRSG) was presented to supplement the existing thermal design process. Flame on a burner and a heat exchanger were postulated as imaginary planes and flame temperature, surface and emissivity was simplified in a aspect of engineering approach. The calculated local flame radiative heat flux on the heating surface was compared with the heat flux of 3-atomic gas radiation and convection.