The objective of this paper was to determine the quantitative bactericidal efficiency of ultrasonic in the root canal. Four test organisms found frequently in the root canal were compared. The result showed ultrasonic can kill the test organisms effectively in the root canals. The best bactericidal efficiency occurred at 4 or 5 min. of ultrasonic though the more efficiency as the time longer. Bactericidal irrigation could increase the bactericidal efficiency of ultrasonic and its bactericidal action similar to irrigation bactericidal property.