In this paper, the stability of alumina-water nanofluid is studied both theoretically and experimentally. The theoretical study is accomplished using the two-phase Eulerian-Eulerian method and the population balance model. The model considers interactions between nanoclusters and their fractal structures to predict size distribution, average size, and settling rate of nanoclusters for different temperatures, fractal structures, and spatial concentrations. The experimental study is accomplished using the dynamic light scattering (DLS) method. Experiments are also performed to validate modeling results. It is found that increased concentration and temperature of the nanofluid lead to increasing aggregation and settling rate of nanoclusters. The results also show that the reducing fractal dimension at certain concentrations of primary nanoparticles leads to an increasing rate of nanocluster aggregation. Generally, the findings of the study indicate that the population balance model can be successfully employed to investigate the stability of alumina-water nanofluid when the interactions between the nanoparticles and the fractal structures of nanoclusters are duly considered. (C) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.