In　this　work,　magnetic　nanocomposite　films　were　prepared　using　magnetorheological　fluids　(MRFs)　and　were　characterized　by　time-domain　terahertz　spectroscopy.　Applying　magnetic　field　during　the　curing　process　of　MRFs　makes　Fe　nanoparticles　align　into　a　chain-like　structure.　Permanent　anisotropy　is　solidified　in　the　films,　resulting　in　a　birefringence　coefficient　as　high　as　0.37　in　a　frequency　range　from　0.4　to　2.0　THz.　The　polarization　state　of　terahertz　wave　can　be　converted　by　rotating　the　film　at　room　temperature　without　external　magnetic　field.　When　the　rotation　angle　is　at　50　$^{\circ}$　–60　$^{\circ}$　,　the　ellipticity　is　greater　than　0.8　in　the　frequency　range　of　1.19–1.6　THz,　indicating　that　the　film　can　be　used　as　a　quarter　waveplate.　IEEE