In this study, a commercially available nano-sized silica (SiO2) was surface-modified via esterification with oleic acid (OA), a relatively inexpensive and hydrophobic modifier, and characterized by FTIR, NMR, SEM, EDS, and TGA measurements. Various amounts of the surface-modified silica nanoparticles (SiO2-OA) were dispersed in a poly(amic acid), which were then cyclized at high temperatures to form a series of PI / SiO2-OA nanocomposite films (PISA). The effect of the addition of the SiO2-OA nanoparticles on the properties of the as-prepared polyimide nanocomposite was studied. The results indicated that, comparing with pure PI and PI / pristine-SiO2 composite film (PISI), the as-prepared PISA films had enhanced dynamic mechanical properties and thermal stability, as well as reduced water absorption and thermal expansion. The as-prepared PI / SiO2-OA nanocomposites have potential for applications in high performance microelectronic devices. (c) 2007 Wiley Periodicals, Inc.