Abstract: To discuss the mechanism of ultrasonic sterilization and provide technical support for the ultrasonic treatment for sludge disposal,the effects of ultrasound power( 250-1 500 W) and frequencies( 25 and 40 k Hz) on the inactivation rate of coliform,cavitation intensity and production of hydroxyl radicals were investigated with the simulated aqueous solution. The mechanism of inactivation was preliminarily discussed. Results showed that under the same ultrasound time and frequency, the inactivation rate of coliform gradually increased with the increases of ultrasound power lower than 1 250 W and then almost the same. The frequency of 40 k Hz showed more superiority than 25 k Hz under the same ultrasound time and power. Cavitation intensity and production of hydroxyl radicals displayed similarity as the inactivation rate did,therefore,the inactivation rate increased with the increases of cavitation intensity and production of hydroxyl radicals. Although the effect of hydroxyl radicals was weak,the oxidation of hydroxyl radicals did play a role in the sterilization that was identified by adding masking agents of sodium bicarbonate( Na HCO₃) into the simulated water.