Polyaniline　nanofibers　with　high　yield　(93%)　have　been　synthesized　by　an　unstirred　polymerization　of　aniline　in　aqueous　acid　media　with　ammonium　persulfate　as　an　oxidant　in　the　absence　of　any　template,　dopant　or　surfactant.　The　morphology,　structure　and　electrical　conductivity　of　the　nanostructural　polyaniline　prepared　under　different　conditions　were　characterized　by　means　of　scanning　electron　microscopy,　FTIR,　UV-vis　and　four-probe　techniques.　It　was　found　that　branched　network-like　polyaniline　nanofibers　were　prepared　without　any　mechanical　stirring,　whereas　irregular　and　rodlike　structural　polyaniline　samples　were　obtained　with　mechanical　stirring　under　the　same　reaction　conditions.　Flowerlike　polyaniline　microspheres　of　ca.　3.5　mu　m　in　outer　diameter,　which　were　constructed　with　nanostructural　polyaniline　lamellar　by　self-assembly　process,　were　synthesized　during　the　polymerization　at　0　degrees　C　for　72　h.　The　as-synthesized　HCl-doped　polyaniline　nanofibers　have　good　room-temperature　electrical　conductivity　of　5.0　S/cm.