The unsteady natural convection flow of a viscous and incompressible fluid through a porous medium with high porosity bounded by a vertical infinite stationary plate in the presence of radiation has been investigated. The fluid is assumed to be a gray, emitting and absorbing radiation, but non-scattering medium. The effects of the radiation parameter, Grashof number and permeability parameter of the medium on the velocity field as well as the effects of the radiation parameter and Prandtl number on the temperature field have been included in the analysis.