One of the common reasons for endodontic failure is incomplete root canal obturation. According to previous studies, about 10-45% of all lower incisors have two canals. The purposes of this study were: 1. determine the incidence of double canals in the lower incisors among Chinese; 2. evaluate the usefulness of X rays for locating double canals in the lower incisors; and 3. propose a method of access cavity preparation for lower incisors when double canals are diagnosed. One hundred lower incisors were collected and checked by dental X ray, mesiodistally, buccolingually and at a 30 degrees mesial shift, in order to determine the incidence of double canals and evaluate their images on film. In addition, a method for drawing the pathway of the double canals from the apex to the incisal edge, as a reference for access cavity preparation, was proposed. The results of this study showed that 36 out of the 100 lower incisors had double canals and only two of them had two separate apical foramina. Clinically, when the X ray image of the lower incisor canal is obscure or missing at the middle third of the root, it is best to extend the access cavity to the incisal edge in order to find the lingual canal.