PURPOSE: To examine the danger zone of mesial root of mandibular first molar of patient without extraction using CBCT(cone -beam computed tomography) to avoid the risk of root perforation. MATERIALS AND METHODS: 20 mandibular first molars without caries and restorations were collected. CT images were obtained by CBCT(PSR9000N TM, Asahi Roentgen Co., Japan), reformed and analyzed by V-work 5.0 (CyberMed Inc., Korea). Distance between canal orifice and furcation was measured. In cross sectional images at 3, 4 and 5 mm below the canal orifice, distal wall thickness of mesiobuccal canal (MB-D), distal wall thickness of mesiolingual canal (ML-D), distal wall thickness of central part (C-D), mesial wall thickness of mesiobuccal canal (MB-M) and mesial wall thickness of mesiolingual canal (ML-M) were measured. RESULTS: The mean distance between the canal orifice and the furcation of the roots is 2.40 mm. Distal wall is found to be thinner than mesial wall. Mean dentinal wall thickness of distal wall is about 1 mm. The wall thickness is thinner as the distance from the canal orifice is farther. But significant differences are not noted between 4 mm and 5 mm in MB-D and C-D. MB-D is thinner than ML-D although the differences is not significant. CONCLUSION: The present study confirmed the anatomical weakness of distal surface of the coronal part of the mesial roots of mandibular first molar by CBCT and provided an anatomical guide line of wall thickness during endodontic treatment.