Results of numerical simulation and experiments carried out to study of natural convection heat transfer of Al2O3/water nanofluids (volume concentrations of 1%, 2%, 3%, and 4%) in a rectangular cavity are presented. The top wall of the enclosure is kept at a higher temperature than the bottom wall, while the other sidewalls are thermally insulated. The numerical study aims to elucidate the influence of microscopic phenomena, including Ludwig-Soret effect (also known as thermophoresis), Brownian motion and sedimentation of nanoparticles, on the natural heat transfer in the enclosure with nanofluids. Four different models are examined in the numerical analysis considering the absence or presence of either sedimentation, or Ludwig-Soret effect. In addition, an experimental investigation is performed on a cavity with the size of 25 mm in width and height, and 60 mm in length to validate the results obtained from the numerical study. The comparisons between experimental data and numerical results unfold that when Ludwig-Soret effect, Brownian motion and sedimentation of nanoparticles are considered in the numerical model, the predicted Nusselt number is very close to experimental data. However, the difference between the Nusselt number obtained from experiments and the numerical study is increased when volume concentration of nanofluids becomes higher than 2%. (C) 2014 Elsevier Ltd. All rights reserved.