Polyimide films in which magnetic Fe3O4 nanoparticles are uniformly distributed are prepared. Before the preparation of the Fe3O4-polyimide composites, pure magnetite nanoparticles (Fe3O4) have been synthesized in water by co-precipitation (from ferric chlorides). Its surface was firstly modified with the 3-aminopropyl triethoxysilane. The prepared polyimide-Fe3O4 nanocomposite films were characterized for their structure, morphology, and thermal behavior employing Fourier transform infrared spectroscopy, scanning electron micrograph, X-ray diffraction, and thermal analysis (DTA/TGA/DSC) techniques.