Highly transparent hybrid films containing nano-sized silica domain were synthesized from acrylic polymer and aqueous monodispersed colloidal silica (CS) with a coupling agent, 3-(trimethoxysilyl)propyl methacrylate (MSMA). The silica content in the hybrid thin films was varied from 0 to 50 wt.%. The experimental results showed that the silica particle size in the precursor solution and the hybrid films was varied from 20 to 40 nm. It could be controlled by the mole ratio of MSMA to silica. The results of scanning electron microscope (SEM), transmission electron microscope (TEM), and elemental analysis support the above conclusion. The prepared hybrid films showed high film uniformity and optical transparence. The thermal stability of the prepared hybrid films increased with the increasing silica content. The refractive index decreased linearly with the increasing silica fraction in the hybrid films. The experimental results suggest that the hybrid thin films have potential applications as passive films for optical devices. (C) 2003 Elsevier B.V. All rights reserved.