This article presents a case with left mandibular first molar with radix entomolaris (RE).It had 3 roots and 4 canals,the distal root with a single canal,the RE with 1 canal.The RE became separate from the mesial root at 4mm below the cementoenamal junction (CEJ),which was classified as “type C separate" variation.Two canals (mesiobuccal root canal and mesiolingual root canal) were presented in mesial root,however the meisiolingual root canal orifice was not detected during the initial treatment period.