In almost all available studies of steady state heat transfer from horizontal thin two-sided plates, the temperatures of the top and bottom surfaces of the plate have been assumed to be the same. The purpose of this study was to determine if, when the temperatures on the top and bottom surfaces of a plate are different, the heat transfer rates, expressed in terms of the Nusselt number, will be different from those that exist when the two surfaces are at the same temperature. Heat transfer results have been numerically obtained for two-dimensional horizontal thin plates and for circular horizontal thin plates for cases where there are different thermal conditions on the upper and lower sides of the plates. Results have only been obtained for the case where the heat transfer from the plate surfaces is to air. The Boussinesq approach has been adopted in dealing with the buoyancy forces and the flow has been assumed to be laminar. The numerical results have been obtained using the commercial CFD solver ANSYS FLUENT^©. The results obtained indicate that, in essentially all cases considered in this study, the difference in the thermal conditions on the two sides of the plate does not produce a significant change in the Nusselt numbers for the surfaces.