The present study considers the thermal structure variation in a porous medium caused by applying the external radiant heating to both the end walls of the porous medium. Besides, the effects of a change in the mass flow rate and the convective parameter are also investigated here. The radiation model is introduced as an unsteady differential form using the two-flux gray radiation model. The reaction zone shifts upstream and the flame thickness decreases either when the porous medium is subject to the external radiant heating, or when the mass flow rate is increased in the low burning rate region. The increase in the convective parameter yields the opposite results.