Abstract: Abstract Objectives To Discuss the best projection angle which can clearly show all the mandibular first molar root canals so as to provide accurate information for root canal treatment. Methods Chose 245 mandibular first molars who needed root canal therapy and had 4 root canals probed with root canal reamer. After root canal therapy, shoot X-ray conventional cast photos,far offset cast photos respectively,and judge their root filling effects. Angles for shooting teeth were conventional 0°,far offset 30°,far offset 45°respectively.The photos were taken by one technologist. The photos were red by four observers independently,diagnosing numbers of root canals within roots and its treatment situations. Results Proportions of X-ray photos at conventional 0 °,far offset 30°,far offset 45°showing four root canals in mandibular first molars were 36％, 82% and 95％respectively.Far offset X-ray projection was obviously better than conventional projection in showing buccolingual canals（P<0.001）. Conclusions Adopting far offset X-ray projection can show root canal numbers in mandibular first molars and their morphology, and evaluate better the root canal situation after root canal therapy. Key words root canal obturation; X-ray projection; mandibular first molar