Abstract: This paper describes three cases of mandibular first molar with three mesial canals. The anatomical variation in the root canal system should be considered during the treatment. The root canal orifices should be carefully explored to decrease the most possible chance of missing extra canal and enhance the successful rate of root canal treatment. The available of dental operating microscope increases the chance to find the mesial canal.

Key words: Mandibular first molar , Mesial canal, Operating microscope