Polyimide (PI)-matrix composite films containing titanium dioxide (TiO2) nanoparticles and layered montmorillonite (MMT) have been fabricated by employing in-situ polymerization and their microstructure has been investigated by synchrotron radiation small angle X-ray scattering, wide-angle X-ray diffraction and scanning electron microscopy. The effects of mixture doping concentration on volume resistivity, loss tangent, permittivity, and breakdown field strength are analyzed. The breakdown field strength of TiO2 and MMT doped PI nanocomposite (PTM) shows a maximum value at the inorganic content of 5 wt.%, which is 10% higher than that of comparable two-component PI/TiO2 nanocomposite films. Meanwhile, the tri-layered PTM/PI/PTM nanocomposite film, prepared by in-situ polymerization, exhibits improved electrical properties than that of the monolayer PTM film. (C) 2013 Elsevier B.V. All rights reserved.