A nanofluid is a colloidal suspension of nanoparticles, and its properties depend on the structures of nanoparticles in a base fluid. In this study, numerical simulations of the dispersion and aggregation behavior of surface-modified Al2O3 nanoparticles in toluene under shear flow are performed. The motion of nanoparticles is calculated based on Newton's equation of motion. Then, the flow of the solvent is determined by solving the Landau-LifshitzNavier-Stokes equation, and both calculations are coupled using the immersed boundary method. The effects of various factors on the dispersion and aggregation behavior of surface-modified nanoparticles are investigated numerically. For nanoparticles modified with oleic acid, which has a high affinity for toluene, the nanoparticles tend to be dispersed in toluene. For decanoic acid, which has a low affinity for toluene, the nanoparticles are dispersed in a limited shear rate range depending on the surface density of the modifier. Such dispersion and aggregation behavior of surface-modified nanoparticles is explained by considering the aggregation due to the high shear flow, which causes the potential energy barrier to vanish, as well as the dispersion of nanoparticles caused by shear stress. (C) 2018 Elsevier B.V. All rights reserved.