Polyimide (PI)-matrix composite films containing inorganic nanoparticles (nano-Al2O3 and nano-TiO2) have been fabricated. A proposed model is used to explain different structures of the (Al2O3–TiO2)/PI (ATP) films synthesized by employing in situ polymerization. Dependences of dielectric permittivities of the ATP films on frequency and temperature were studied. Results show the breakdown strength of the films decreases with prolonging the corona aging time. The incorporation of the nano-Al2O3 and nano-TiO2 particles significantly improves the corona resistance of the films. The corona aging also influences the infrared absorbance, the glass transition temperature (Tg), and loss factor (tanδ) of the ATP films.