Analysis of combined natural convection with surface radiation in a two-dimensional enclosure is carried out. To search the optimal location of the heat source, the entropy generation minimization approach and conventional heat transfer parameters are used and compared. Air is considered as an incompressible fluid and transparent media filling the enclosure with a steady and laminar regime. The enclosure internal surfaces are also gray, opaque, and diffused. The governing equations are solved using the finite difference approach. The results show that with increase in emissivity, entropy generation decreases, and as the Rayleigh number increases, the rate of entropy generation increases. Furthermore, optimum design with the maximum dimensionless temperature and convective Nusselt number confirms the applicability of the second law for optimal design. K E Y W O R D S constant flux heat source, entropy generation minimization, natural convection, optimal search, surface radiation