In this paper the problem of coupled radiation and natural convection is investigated in a square porous cavity using local thermal non-equilibrium model. The radiative transfer equation (RTE) is solved by the discrete ordinates method (DOM) and the energy and momentum equations are solved using finite element method. The results of the present study are compared with that of the other investigations which have used another method to solve radiative transfer equation. Effective parameters on heat transfer and fluid flow characteristics such as Planck number, inter-phase heat transfer coefficient and scattering albedo are studied and the results are presented in the group of dimensionless parameters. The results indicate that the solving method of the radiative transfer equation would have significant effect on the fluid flow and heat transfer characteristic. In the case of low optically thick media, discrete ordinates method (DOM) is more precise than the other methods which used in other literature.