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. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104, 4096–4105, 2007