Combined natural convection and thermal radiation heat transfer in air-filled square gray enclosure are numerically investigated for three different cases having the same constant temperature difference. The partially heated patch of the left wall and the cold right wall are maintained at constant temperatures while the other walls are insulated. The surface radiation heat transfer is considered from the heat sources (patch) to the other surfaces, and the medium is assumed to be nonparticipating. The emissivity ratio of the walls are kept the same. The transport equations along with Boussinesq approximation as well as the energy equation are solved using FLUENT software. As the wall emissivities were changed, the steady state mean Nusselt numbers over the cold surface were computed for each case as a function of the Rayleigh number. In this study, the Rayleigh number was ranged from 10(5) to 10(7) while the surface emissivity ratio was changed from epsilon=0 to 1. The heat transfer and fluid flow characteristics were investigated with respect to the effect of the conduction-radiation parameter, Rayleigh number and the wall emissivity.