A numerical analysis has been conducted to examine combined radiation-convection-conduction heat transfer over a backward facing step flow. LU-SSOR scheme and Finite Volume Method was used to solve compressible Navier-Stokes equation and energy equation, while the radiative transfer equation was solved by Discrete Ordinates Method(DOM). The thermal behavior has been influenced such that the fluid was heated up faster due to the radiation. Furthermore, the reattachment length was also shrieked because of the reduced adverse pressure gradient. However, the radiative heat flux was found to play some role in the recirculating zone only.