Magnetic nanoparticles have attracted intensive attention for their wide applications as biomaterials and magnetic storage materials. Polyurethane is one of the most biocompatible polymers and has been used widely in vivo. In this paper, the magnetite nanoparticles were synthesized by chemical precipitation under different conditions. The as-prepared samples were characterized by X-ray diffraction and transmission electron microscopy, and their magnetic properties were evaluated on a vibrating sample magnetometer. Then the magnetite nanoparticles with different amounts were doped into polyurethane directly and composite films were made. Reinforced by the inorganic particles, PU composite films were also characterized by Fourier transform infrared spectra and mechanical tests., and the surface morphology of the composite film was observed by Atomic Force Microscope The results showed the composite material was reinforced by magnetic nanoparticles and also showed magnetic behavior. This kind of composite materials have the potential to be used as hyperthermia treatment in biomedical field, like coatings on cardiovascular stents.