Mixed-convection fluid flow and heat transfer in a square cavity filled with Al2O3-water nanofluid have been numerically investigated using the finite volume method (FVM) and SIMPLER algorithm. The geometry is a lid-driven square enclosure with an interior rectangular heated obstacle. The bottom, top, and left walls are adiabatic while the right wall has a constant low temperature (T-c). The upper lid of the cavity moves in a positive direction. Effects of different key parameters such as Richardson number, position and height of the block, the volume fraction of the nanoparticles, and cavity inclination angles on the fluid flow and heat transfer inside the cavity are investigated. It is observed that the average Nusselt number for all ranges of solid volume fraction increases with a decrease in the Richardson number. The results elucidate irregular changes of mean Nusselt number at different Richardson numbers versus variation of inclination angles in any case. (C) 2015 Elsevier Ltd. All rights reserved.