In the current study, a numerical investigation of two-dimensional combined convection-radiation heat transfer of turbulent gas flow over a backward-facing step (BFS) in a horizontal rectangular duct is presented. The computational domain contains two different parts including gas flow and solid element that makes the problem as a conjugate one. The gas phase is considered to be a radiating media that can absorb, emit and scatter thermal radiation, where in solid phase, heat transfer takes place by conduction. The set of governing equations for gas flow is solved numerically using the CFD technique and the model is employed for computation of turbulence fluctuations. To evaluate the radiative term in the gas energy equation, the radiative transfer equation (RTE) is solved by the discrete ordinates method (DOM). Inside the solid phase, the conduction equation is solved to obtain the temperature distribution. The effects of conduction ratio, optical thickness, radiation-conduction parameter and albedo coefficient on heat transfer behavior of the system are carried out.