To evaluate the use of dental operating microscope (DOM) and ultrasonic instruments in treating blocked canals.135 blocked canals in 92 teeth were treated with ultrasonic tips and files under DOM. The etiology of canal blockage included calcification, broken instruments, post, resinifying therapy or ledge. These canals were negotiated and root canal therapy or pretreatment was performed. The success rates were calculated.77 teeth were successfully managed with a success rate of 83.7%. 84.4% of the blocked canals were negotiated. The success rates of each category of the blocked canals were: 88.1% for calcified canals; 76.9% for canals blocked by instrument fragments; 100% for canals blocked by post; 81.1% for canals subjected to resinifying therapy; and 78.6% for ledged canals. Complications such as root canal perforation or vertical fracture did not occur.The use of DOM and ultrasonic instruments is proved to be an effective way in the management of blocked canals.