The onset of thermal convection in a translucent inclined porous layer in considered. Attention is focused on the effects of radiative heat transfer and inclined angle on the critical Rayleigh-Darcy number and the convection cell size. If we consider the contribution of radiative heat transfer, the basic temperature profile is non-linear and the thermal convective instability is influenced by the ratio of conduction to radiation heat flux, the temperatures at the boundary surfaces and radiative parameters such as wall emissivity(εω),scattering albedo(ωτ) and extinction coefficient(βτ) as well as the usual Rayleigh-Darcy number. In the present work, effects of inclination angle and radiative heat transfer on the basic state temperature profile and onset of convective instability are investigated with the help of linear stability theory employing Darcy’s law and the radiative transport equation simplified by the P1 approximation. The increased effective thermal conductivity due to radiation inhibits the onset of convection and causes higher critical Rayleigh-Darcy number and smaller convection cell size. As the inclination angle increases, the effective buoyancy force decreases and the heating rate needed to induce convection increases due to the increased flow rate, resulting in larger critical Rayleigh-Darcy number and smaller critical wave number.