This paper is a numerical study was conducted to investigate the conjugate of the flow and heat transfer from the three-dimensional natural convection, in a cubic enclosure partially filled with a central cubic porous block which is in out thermal equilibrium with the fluid media. The physical model considered here assumes the existence of a temperature difference across the enclosure between the left and the right wall, the other walls are adiabatic. Under these conditions, flow from inside the enclosure is generated by the temperature difference across the enclosure and the interaction between the solid matrix and the fluid. Variations of Nusselt number on the hot and cold walls are also presented to show the overall characteristics of heat transfer to the interior of the enclosure. The study found that the fluid flow and heat transfer are governed by the dimensionless thickness of the porous layer , and the thermal conductivity ratio of the solid matrix of the porous media to that of the fluid . The complex obtained flow structure and corresponding heat transfer (velocity, temperature profiles) are discussed at a steady state. The numerical results are reported in terms of isotherms, velocity field, streamlines, and averaged Nusselt number. Thus, the results of this work can help develop new tools and means to optimize the overall heat transfer rate, which is important in many electronic energy components and other systems.