At high temperature the effective thermal conductivity of a bulk material with a large temperature difference may differ from the local equivalent thermal conductivity of a unit cell at the mean temperature due to radiation. This paper analyzed the local equivalent thermal conductivity and the effective thermal conductivity for porous materials with cylindrical and spherical air cavities. Prediction for a spherical case agrees well with experimental data. Investigation shows that the equivalent thermal conductivity at mean temperature can well represent the effective thermal conductivity for a cavity diameter of < 5 mm and porosity < 78% although the local equivalent conductivity can vary enormously.