In many compartment natural-convection problems associate with functional design of heat transfer for ships and ocean structures, large temperature difference occurs between the surrounding walls. Examples of such heat transfer problems include a cabin of a passenger ship when the neighboring cabin is on fire, and reefer holds of a refrigerating cargo ship. In these situations, heat radiation exchange among the wall surfaces cannot be neglected if the wall emissivities are not very small. In this paper, a numerical study is reported on combined turbulent convection, heat radiation and conduction in a rectangular enclosure with all six surrounding walls having finite emissivity and conductance. The turbulent convection is computed by a large eddy simulation (LES) method. The emphasis of the study is on the effects of wall radiation exchange on turbulent natural convection in a rectangular enclosure. For comparison with the computation, a laboratory experiment is also carried out on a rectangular enclosure with one sidewall heated by electric heaters. The effect of wall radiation on the natural convection is investigated by changing the wall emissivities. Excellent agreement of temperature between numerical predictions and the experimental measurements is obtained.