Polyimide/BaTiO3 composite films with different BaTiO3 doping concentrations have been successfully fabricated by in-situ polymerization method and their microstructures were studied by synchrotron radiation small angle X-ray scattering (SAXS) technique. Analyzed SAXS results indicate that the average radius of the nanoparticles in nanocomposite films distribute from 30 nm to 42 nm, which are consistent with the observation from a Scanning Electron Microscope. The positive deviation of SAXS results indicates that there is a fluctuation of electron density in the materials and the distribution of inorganic is not uniform. The dielectric constant and loss tangent increase with the BaTiO3 doping concentration increases, and the breakdown strength decreases. (C) 2013 Elsevier B.V. All rights reserved.