Use of an ultrasound generator as an emulsifier leads to efficient initiation in polymerization processes. In this study, polymerization of styrene under indirect ultrasonic irradiation was carried out without a chemical initiator. The resulting latex particle size was around 1.0 mu m and the weight-average molecular weight was in the range from 2,000,000 to 8,000,000. OH radical generation inside the reactor was evaluated by KI oxidation dosimetry and found to depend on a dimensionless index (1+h/2)/lambda, where lambda is the wavelength. The region of low generation of OH radicals overlaps with the region of low, polymerization rate, where the value of (1+h/2)/lambda is about 0.6. Therefore, KI dosimetry by using (1+h/2)lambda is useful for evaluation of the chemical efficiency of the polymerization. In addition to radical generation, the stability of emulsion was also found to be important for the polymerization under ultrasonic indirect irradiation.