Polystyrene latex particles were synthesized using a method based on emulsifier-free miniemulsion polymerization under ultrasonic irradiation in the presence of 2,2' azobis (2-amidinopropane) dihydrochloride (V-50) as a cationic ionizable water-soluble initiator and cetyl alcohol as costabilizer. The optimized conditions were obtained by using various parameters, such as the amounts of monomer and initiator, and the time and power of ultrasonic irradiation. In optimal conditions, the latex particles appeared to be about 250 nm in diameter through scanning electron microscopy (SEM). The SEM and gel permeation chromatography (GPC) analyses and monomer conversions of emulsifier-free miniemulsion polymerization were compared with those of conventional emulsifier-free emulsion polymerization using V-50 as initiator in both cases. The results showed that in the miniemulsion polymerization, the rate of polymerization (Rp) was significantly higher, and latex particles were significantly smaller than those in the conventional emulsion polymerization. (c) 2007 Wiley Periodicals, Inc.