In this investigation, steady tow-dimensional natural convective heat transfer in rectangular porous cavity, (heated from below) with horizontal walls heated to uniform but different temperatures and adiabatic sides has been studied numerically. The numerical results of heat transfer rates are presented for porous Rayleigh numbers (Ra*), based on width of cavity, in the range (Ra*≤500), with layer aspect ratios (Ar), (height / width) ranging between (0.5≤Ar≤5). Plots of streamlines and isotherms to show the behavior of the flow and temperature distribution are presented. The current study shows that the Nusselt number is a strong function of the porous Rayleigh number, and the geometry of the cavity is represented by aspect ratios. Porous Rayleigh number has a large effect on the flow field, whereas any increase in (Ra*) results in changing the flow pattern from unicellular to multicellular flow. Correlation equation has been obtained to show the dependence of Nusselt number on the porous Rayleigh number, and aspect ratio (Ar), as this correlation will be beneficial in design of systems of thermal insulators in the energy storage engineering applications.