The effects of ultrasonic wave irradiation on bacterial inactivation were investigated as functions of the ionic strength (IS), humic acid, and temperature. Escherichia coli (E. coli) D21g was selected as a model bacterium to better catch the effect of three parameters on the cell inactivation behavior. The Suwannee River humic acid (SRHA) was chosen as a representative humic acid, and the concentration for ultrasonic tests was kept to 10 ppm. The frequency of ultrasonic wave employed was 20 kHz, and the inactivation efficiency at two exposure times (5 and 10 min) was compared. The removal efficiency of E. coli D21g was confirmed to be 100 % at 10 min in all conditions except the 10-min temperature-controlled condition. The removal efficiency with the high IS was greater than that with the low IS, by 26 %, confirming an increase in the bacterial inactivation level with increasing IS. The bacterial removal efficiency with SRHA (96.6 %) was much greater than that without SRHA (69.6 %). The removal efficiency in the temperature-controlled condition (at a relatively low temperature) was significantly lower than that in the uncontrolled condition. Furthermore, the trend obtained using two other types of bacteria with more complex surface structure was consistent with that using the E. coli D21g cells.