A numerical study of natural convection flow and heat transfer of Copper (Cu)-water nanofluid inside an eccentric horizontal annulus is presented. The inner and outer cylinders are kept at constant temperatures as T-h and T-c, respectively. First the governing equations in terms of stream function-vorticity formulation in polar coordinate system for eccentric physical domain are derived and then transformed to a rectangular domain in order to get better accuracy of the solution near the boundaries. The resultant governing equations are discretized with a finite volume technique based on second order upwind scheme and then solved by iteration. The effects of the eccentricity epsilon, radii ratio (RR), the nanoparticles volume fraction parameter phi, the Rayleigh number Ra and the PrandtI number Pr on the mean Nusselt number Nu, streamlines and isotherms are investigated. The results are also discussed in detail. It is found that a very good agreement exist between the present results and those from the open literature. (C) 2013 Elsevier Ltd. All rights reserved.