A two-dimensional steady laminar natural convection in an inclined rectangular enclosure filled with CuO-water nanofluid is investigated numerically. The horizontal walls are thermally insulated and the left vertical side wall is heated by a spatial temperature distribution. Mass Conservation, momentum, and energy equations are solved numerically by the finite volume element method using the SIMPLER algorithm for pressure velocity coupling. This study has been carried out for the pertinent parameters in the following ranges: inclination angle omega = 0 degrees-90 degrees, the volumetric fraction of nanoparticles between 0 and 4% and aspect ratio (Ar = 0.25, 0.5, 1 and 2). These simulations are performed at constant Rayleigh and Prandtl numbers, they are fixed at Ra = 10(5)and Pr = 7.02. The results are presented in the form of streamlines, isotherms and Nusselt numbers. The heat transfer increases first, then decreases with increasing the inclination of the enclosure for aspect ratio (Ar >= 1) and increases with increasing the inclination angle for Ar < 1. The rate of heat transfer increases with increasing of volume fraction of nanoparticles. The subject of this paper is to study the effect of nanoparticles on heat transfer, together with the effect of the tilt angle and the aspect ratio. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.