Polyaniline (PANI) nanofibers were chemically prepared through ultrasonic irradiated polymerization with varying ultrasonic power, frequency, and reaction temperature. It was found that PANI nanofibers with smoother surfaces and uniform diameters can be achieved by increasing the ultrasonic power or the reaction temperature in the studied ranges; a higher reaction temperature was also beneficial for producing PANI nanofibers with larger aspect ratios. With the ultrasonic power set to 250 W, although the polymer prepared at higher frequencies showed higher purity as well as smoother surfaces than those at lower frequencies, the one prepared at 50 kHz with uniform diameters of about 80 nm and lengths of about 700 nm performed best. With the ultrasonic power and frequency fixed and aniline polymerization carried out between 0 and 75 degrees C, PANI nanofibers exhibiting larger aspect ratio and less agglomeration were obtained under a higher reaction temperature. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 868-875, 2009