Abstract: Abstract：0bjective To explore clinical effects of combining dental operating microscope with ultrasonic technique on treating difficult root canals. Methods 74 teeth of various kinds of 112 difficult root canals were selected and treated with Microscope and ultrasonic technique. The clinical effects and the success rate after six months were evaluated. Results：55 patients completed root canal therapy and got clinical success with 74.32% success rate; in terms of root canals, there were 81 successful root canal treatments with success rate of 72.32%. After six months the rate was 82.69%. The overall success rate was 60.56%. Conclusions Microscopic ultrasonic technology application provides an effective means for difficult root canal treatment.