Poly(3,4-ethylenedioxythiophene) (PEDOT) nanoparticles with a size of 50-100 nm were prepared by the oxidation polymerization of the monomer 3,4-ethylenedioxythiophene (EDOT) under ultrasonic irradiation. The structure and morphology of PEDOT, as well as the electrical conductivity were characterized. Compared with the conventional stirring method, the PEDOT nanoparticles prepared by ultrasonic irradiation have a smaller particle size and are more uniform and spherical due to the dispersion, emulsifying and disruption effects of ultrasound. The yield of PEDOT by ultrasonic irradiation can reach similar to 50%, and the conductivity of PEDOT is high up to 11 S/cm. A decrease in particle size due to the ultrasonication can lead to more effective doping and enhanced conductivity. The reaction time, temperature and ultrasound power output have significant effects on the field and electrical conductivity of PEDOT. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 2146-2152, 2010