Research Object: Identify anatomy of the root canal system of the first molar teeth mandible. Rresearch method: It is a descriptive horizontal section study with 19 of the first molar teeth mandible which were casted resin blocks and cut into 5 slices: (Slice 1: From tip 1mm. Slice 2: From tip 2 mm. Slice 3: From tip 3 mm. Slice 4: Through 1/3 between of root teeth. Slice 5: Over 1/3 above of root teeth). Research results: The near - far dimension: Dimension of near root teeth similar to far root teeth. The internal - external Dimensions: Dimension of near root teeth are larger than the far root tooth. The dimensions increase through the slices: The near root teeth: Type IV is majority, there are not type I, III and type V. The far root teeth: Type I is majority, there are not type V. The near root teeth: The ratio has two canals is 100 percent. The far root teeth has two canals is 37 percent. Branching is the most in oral canal root teeth, except a few far root teeth (type III = 10,5 percent). Most of the near and far root teeth have foramen at central position. The ratio foramen of far root teeth is skewed toward the far is 10,5 percent. Conclusion: The canals have tapering. The taper of canal similar to taper of root teeth. In the first slice: Dimension of canal is the smallest. The ratio has 2 canals in the near root teeth is 100 percent and the far root teeth. The far root teeth has two canals is 37 percent. Most canals have circular and oval structure. Most of the near and far root teeth have foramen at central position. The ratio foramen of far root teeth is skewed toward the far is 10,5 percent.