To explore the use of dental operating microscope with ultrasonic instruments in root canal retreatment.164 root canals in 124 teeth of 111 patients who need root canal retreatment were treated under dental operating microscope (DOM) with ultrasonic instruments, then the success rates were calculated.Both 136 root canals with a success rate of 82.93% from 164 root canals and 103 teeth with a success rate of 83.06% from 124 teeth were managed successfully. The each category of the retreated teeth was 85.29% for calcified canals, 83.33% for canal subjected to resinifying therapy, 86.67% for misunderstood root canals, 100% for canals blocked by post, and 66.67% for both root canals blocked by instrument fragments and ledged.The use of microscope and ultrasonic instruments was proved to be an effective way in the root canal retreatment.