The present contribution is a numerical investigation of the natural convection between two circular objects, where the first one is hot and the second is cold. Both cylinders are placed in an enclosed adiabatic cavity filled with copper-water nanofluid. The cylinders are arranged horizontally in the middle of cavity height. The main target of this paper it to examine the effects of geometrical configurations and the thermo-physical characteristics of nanofluid on the fluid motion and heat transfer rates. The study is conducted for various parameters: Rayleigh number (Ra = 103 to 105), nanoparticle volume fraction (φ = 0 to 10%), the diameter of cylinders (d/H = 0.2 to 0.4), and the gap spacing between cylinder (S/H = 0.25 to 0.7). Also, two cavity shapes are studied (square and circular). The average Nusselt number of the cylinders is computed and plotted as function of the studied parameters. It was concluded that the cavity shape and the particle volume fraction have a negligible effect on the heat transfer rate, whereas the distance between cylinders and the cylinder diameter have a remarkable effect on the flow patterns and convective heat transfer.