In this study, a numerical simulation of the thermal performance of two ribs mounted over a horizontal flat plate and cooled by Cu-water nanofluid is performed. The plate is heated and maintained at a constant temperature and cooled by mixed convection of laminar flow at a relatively low temperature. The top wall is considered as an adiabatic condition. The effects of related parameters such as Richardson number (0.01 ≤ Ri ≤ 10), the solid volume fraction (0.01 ≤ ϕ ≤ 0.06), the distance ratio between the two ribs (d/W = 5, 10, and 15), and the rib height ratio (b/W = 1, 2, and 3) on the ribs thermal performance are studied. The numerical simulation results indicate that the heat transfer rate is significantly affected by the distance and the rib height. The heat transfer rate is improved by increasing the nanoparticles volume fraction. The influence of the solid volume fraction with the increase of heat transfer is more noticeable for lower values of the Richardson number. The numerical results are summarized in the effect of pertinent parameters on the average Nusselt number with the assistance of both streamlines and isothermal ones. Throughout the study, the Grashof and Prandtl numbers, for pure water are kept constant at 103 and 6.2, respectively. The numerical work was displayed out using, an in-house computational fluid dynamic code written in FORTRAN, which discretizes non-dimensional forms of the governing equations using the finite volume method and solves the resulting system of equations using Gauss-Seidal method utilizing a tri diagonal matrix algorithm.