This paper investigates suitable approximation for Calculating the thermal radiation flux divergence and effect of errors on performance evaluation of porous radiant burners (PRB).Thus, a single layer and a double layer of buried flame type of the porous radiant burners have been selected and numerically simulated. Due to the significant difference in the temperature of the solid matrix and the fluid passing the burner, the energy equations was considered as a non-local thermal equilibrium. Complete kinetics of methane air was used for combustion modeling. Since the effect of lateral walls should be neglected the problem was solved in 1D to present exact solution of RTE and compares the other approximations. Results show that discrete ordinate as well finite volume approximation of RTE show that eight directional spherical split is the best selection. Lower ordinates have substantial deviation and increasing the number of division enlarges computation cost without any considerable improvement on errors reductions. Furthermore, two flux method and Rosseland approximation are not valid for this kind of modeling.