Abstract : The purpose of this study was to investigate the root canal morphology of extracted second mandibular molars determined as single root. Sixty-nine single rooted mandibular second molars were used. Gutta-percha point was inserted into the root canal after root canal preparation by a standardized method. Transparent specimens were made from the test teeth and the root canal morphology was evaluated on images taken with a digital microscope. A C-shaped canal was found in 28.9%. When the root canal was prepared in accordance with the root canal orifice observed in the pulp chamber floor, a missed canal was found in 42% of the teeth. Missed canals could not be discovered due to branching inside the root.