Polypyrrole　nanoparticles　with　a　high　yield　(93%)　were　synthesized　by　using　unstirred　polymerization　in　the　absence　of　any　template　or　surfactant.　The　structure,　elctroconductivity　and　morphology　of　the　polypyrrole　nanoparticles　were　characterized　by　FT-IR,　wide　angle　X-ray　diffraction,　four-probe　techniques,　scanning　electron　microscopy　and　transmission　electron　microscopy　(TEM).　The　particle　size　and　morphology　of　polypyrrole　nanoparticles　were　systematically　studied　by　changing　synthetic　conditions,　such　as　oxidant　type,　molar　ratio　of　ammonium　persulfate　(APS)　to　pyrrole,　polymerization　temperature　and　reaction　medium　species.　It　was　found　that　the　particle　size　and　morphology　of　the　polypyrrole　nanoparticles　varied　significantly　with　the　above-mentioned　polymerization　parameters.　Using　different　synthetic　conditions,　we　can　control　the　diameters　of　polypyrrole　nanoparticles　ranging　from　40　nm　to　260　nor.　The　preparation　conditions　of　ammonium　persulfate　as　an　oxidant,　APS/pyrrole　molar　ratio　of　0.25,　at　a　polymerization　temperature　of　0　degrees　C　are　optimal　for　the　unstirred　polymerization　synthesis　of　polypyrrole　nanoparticles.　TEM　images　revealed　that　uniform　polypyrrole　nanoparticles　in　diameters　of　43　nm　were　prepared　in　1.0　mol/L　HCl　aqueous　medium　at　0　degrees　C　(using　an　ice　bath)　with　APS/pyrrole　molar　ratio　of　0.　25.　The　room-temperature　electroconductivity　of　the　hydrochloride　doped　polypyrrole　disk　is　up　to　5.5　S/cm.