Hierarchical　polyaniline/polypyrrole　(PANI/PPy)　copolymer　nanofiber　was　prepared　via　a　two-step　method　and　adopted　as　dispersing　materials　for　electrorheological　(ER)　fluids.　The　first　step　was　used　to　synthesize　PANI　nanofibers　by　a　rapid　mixing　method.　Subsequently,　the　PANI/PPy　copolymer　nanofibers　with　a　rough　surface　were　obtained　using　an　in　situ　polymerization　method　continuously.　The　morphology　of　the　resultant　PANI/PPy　copolymer　nanofibers　can　be　controlled　by　varying　the　amount　of　Py　monomer　in　the　secondary　in　situ　polymerization　method.　The　rough　surface　of　PANI/PPy　copolymer　nanofibers　were　confirmed　by　scanning　electron　microscopy　and　transmission　electron　microscopy.　The　diameter　of　PANI/PPy　nanofiber　is　within　the　range　100-200　nm.　The　obtained　PANI/PPy　copolymer　particles　all　exhibit　amorphous　structure　through　X-ray　diffraction　measurement.　We　also　demonstrated　that　the　hierarchical　PANI/PPy　copolymer　nanofibers　exhibited　characteristic　ER　behaviors,　which　were　investigated　using　a　Haake　rotational　rheometer　at　various　electric　field　strengths.　The　ER　efficiency　e　for　PANI-1mLPPy　and　PANI-2mLPPy　ER　fluids　at　shear　rate　0.1　s(-1)　is　36.6　and　28.5　under　electric　field　strength　E=3　kV/mm,　respectively.　Low　leaking　current　density　is　observed　even　at　high　electric　field　strength　and　wide　plateau　region　appeared,　which　show　a　strong　ER　activity　for　the　PANI/PPy　composite　nanofibers.　The　results　also　indicate　that　the　PANI/PPy　composite　particles　have　distinctly　enhanced　ER　effect　compared　with　the　pure　PANI　and　PPy　particles　under　electric　stimuli.　The　significantly　improved　ER　property　of　PANI/PPy-based　ER　fluid　is　ascribed　to　the　enhanced　interfacial　polarization.　(c)　2018　Wiley　Periodicals,　Inc.　J.　Appl.　Polym.　Sci.　2018,　135,　46289.