Objective To estimate the diagnostic potential of the dental operating microscope used for locating the second mesiobuccal canal（MB2） orifice in maxillary first molars. Methods 120 extracted maxillary first molars were used, all the access cavities were prepared and fixed on the dental study model. MB2 orifices were examined with naked eye and under a dental operating microscope（DOM）. The examination was repeated by another dentist with the same methods. Then Indian ink was injected into the pulp chamber and all the teeth were treated to be transparent to determine if it had a real MB2. Results The sensitivity and accuracy of the DOM group were 0.78 and 0.76, higher than 0.61 and 0.65 of the naked eye group（P
