Statement of Problem: Complete knowledge of the morphologic and microscopic anatomy of the dental root apex has been recognized. Internal root canal anatomy especially root apex is variable in human teeth for example lateral canal and ramification may exist.Purpose: The objective of this study was to determine the internal anatomy of root apex.Materials and Method: For this investigation, 50 mandibular first premolar and 50 mandibular second premolar were obtained. The age and sex of the patients and reasons for extraction were not recorded. For better recognition of apical foramen and lateral foramen of the teeth, they were placed in methylen blue for 2 hours and then teeth were studied under the stereomicroscope with ×25 magnification lens (the calibration of lens was 0.1mm) and percentage of lateral canal, root deviator and apical foramen exit were detected. Then, to study the internal anatomy, the teeth were grinded and stained with hematoxylin. The mean, standard deviation and standard error for the diameter of constriction and distance from apical foramen were measured. Also, the internal anatomy of the root canal was studied by two observers.Results: The results showed that apical constrictions were always smaller than apical foramen and a distance existed between apical constrictions, apical foramen and vertex. Apical foramen did not exist in center in 80 to 90% of cases. Lateral canals were observed in 28% of mandibular first premolar and in 34% of mandibular second premolar. Also, 90% of first premolar teeth and 88% of second premolar teeth had one canal and 10% of first premolar and 12% of second premolar had two canals.Conclusion: As the mean distance between apical constriction and apical vertex in mandibular first premolars was 0.65mm and in mandibular second premolars was 0.68mm, therefore during root canal treatment and working length determination, this distances should be decreased from tooth length.Key words: Root canal anatomy, Apical foramen, Apical constriction