The mandibular first premolar has complicated morphology of root canal, it has more variation which increases the incidence of multiple root canals. A successful root canal therapy not only need strict cleaning, disinfection and filling, but also need a correct understanding of root canal morphology and anatomy. Otherwise there might be a risk of missing canals that resulting the failure of the therapy. Also crown closure is one of the successful elements of root canal therapy. This is a case report accompanied by a review of the literature that shows the unusual anatomy of the mandibular first molar with three roots and three root canals.