SiO2 nanoparticles modified by oleic acid (OA) were prepared using surface-modification method in this work. Infrared spectroscopy (IR), transmission electron microscopy (TEM) and X-ray photoelectron spectra (XPS) were used to investigate the structure of the modified SiO2 nanoparticles. Effects on coverage and dispersion in oil by OA concentration were also studied. The results indicate that OA-modified SiO2 nanoparticles are capable of dispersing stably in mineral oil and the OA is bonded to the surface of SiO2 nanoparticles through esterification. The optimal reaction temperature and the proportion of OA to SiO2 were established. (C) 2003 Elsevier Science B.V. All rights reserved.