The aim of the present study was to compare the sizes of the first instrument with or without taper that bind to the narrow apical diameter of the root canals after coronal flaring. One hundred thirty five human extracted various teeth groups with intact apices were examined. Standard endodontic access cavity was performed and the coronal third was flared using gates glidden reamers sizes 1-3. Apical patency was established using K-file size 10 and working length were determined. Non tapered instruments (Lightspeed® ) were gently introduced by hand to each canal in ascending order starting with size 20 to working length. The first instrument that bound at the canal walls and reached the working length was recorded as FLSB. Then standardized K-Files hand instruments were gently introduced to working length starting with size 15 and the first K-file that bound to the canal walls and reached the working length was recorded as FKFB. Statistical analysis was performed using t-test. The results showed that the average size of FLSB was larger in approximately 1-2 ISO sizes than FKFB. The minimal differences was found in D root of mandibular molars (4.6±2.8) and the maximal differences in the MB root of maxillary molar 9.2 ± 3. It is concluded that the first non-tapered instruments to bind the apical constriction are larger than the tapered instruments and probably reflects better the actual narrow apical diameter of the canal.