Polyimide (PI)/Titanium dioxide (TiO2) nanocomposite films were prepared by in-situ polymerization method and sandwich structures (PI-TiO2/PI/PI-TiO2) were obtained. The microstructures, as well as the electrical and mechanical properties of hybrid films were characterized and analyzed. The results indicate that the TiO2 nanoparticles were homogeneously dispersed in the hybrid films. Among all the tested samples, the electrical insulation and mechanical properties of tri-layered PI/TiO2 nanocomposite films with 5 wt.% doping concentration are the best. The breakdown field strength and volume resistivity of PI/TiO2 composite film with sandwich structures are 216.5 (kV/mm) and 0.847 x 10(17) (Omega cm), which are 8% and 20% higher than those of monolayer PI composite film, respectively. (C) 2013 Elsevier B.V. All rights reserved.