The aim of this study was to investigate the shapes and diameters of the physiological foramen and anatomy of the root canal at 3mm from apex in mandibular first molars.

Sixty mandibular first molars were randomly selected. The apical anatomy of 60 mandibular first molars was investigated by means of a stereomicroscope (60x magnification).

The results were as follows;

1. There was a high percentage of two physiological foramina in mesial (61.67%) and one foramen in distal(71.66%) roots of mandibular first molars.

2.There was a high frequency of accessory foramina in mesial roots with one foramen (26.07%).

3. The diameters of physiological foramen was as follows:

0.329mm in single mesial foramen

4. The most common physiological foramen shape was oval (69.93%).

5. The incidence of isthmus in mesial root at 3mm from apex was 55%. The 3mm-sections contained a complete isthmus 31.66% and a partial isthmus 23.34%.

6. 3mm from the apex, the most common canal shape was oval (50.64%).

Knowledge of the apical anatomy of mandibular first molar would be necessary for success of surgical and nonsurgical endodontic treatment.