Abstract: The formation of free radicals ·OH and ·H in a naturally air-saturated aqueous solution exposed to therapeutic CW ultrasound at the frequency of 820 kHz has been confirmed by using spin trapping 5, 5-dimethyl-1-pyrroline-1-oxide (DMPO) and electron spin resonance (ESR) technique. It is suggested that these radicals are formed due to the high temperature and pressure produced by the ultrasonic transient cavitation The transient cavitation threshold is found at 0.537w/cm²-0.632w/cm² under the sonifi-cation time of 3 minutes. With increasing sound intensity the yield of free radicals ·OH raises rapidly at the intensity ranging from lw/cm² to 2w/cm², and no more increase is observed above 3w/cm². The sound intensity (I) dependence of the yield of ·OH (D) can be approximately described by a regression equation: D = √I-√I_C）^1/2， where I_c=0.667 w/cm². Under fixed sound intensity the yield of ·OH increases monotonously with the sonification time.