Objective: This study compared retrievability of two rotary nickel Titanium (NiTi) instruments with symmetric versus asymmetric cross sections; ProTaper Universal® and RevoS®, respectively, with reference to the influence of some anatomic factors, as well as evaluated of the remaining minimum dentin thickness after retrieval using cone beam computed tomography (CBCT). Materials and Methods: Thirty six extracted mandibular first molars with curved mesiobuccal canals were used. The apical 4mm of ProTaper Universal rotary NiTi instrument ® (Dentsply Maillefer, Ballaigues, Switzerland) size F3 (n=18) and RevoS rotary NiTi instrument ® (Micro- Mega, Besanc¸on, France) size A35/06 (n=18) were fractured in the apical region of the canal. Fragments were located at apical or apical-middle and their coronal end were before canal curvature, at or beyond it. Microsonic technique was successful when instrument retrieval occurred without mishaps. Post retrieval CBCT was superimposed on preoperative CBCT to determine changes in distal dentin thickness (at coronal end of instrument (C level) and at furcation level (F level) and in canal width. Chi-square test evaluated success incidence between the two instruments types and effects of influencing anatomic factors. Mann-Whitney U test compared quantitative data, significance was set at P<0.05. Results: Overall success rate for removal of fractured fragments was 72.2% with equal incidence in both instruments types. Retrieval of ProTaper instrument was positively affected by being in the apical-middle location, P=0.047. Retrieval of RevoS instrument was positively affected by being before and at the curvature; P = 0.04. CBCT measurements showed overall minimum distal dentin thickness after retrieval to be approximately 0.67±0.14 mm. Percent of dentin removal was significantly higher at F level, P=0.04, while Canal width was higher at C level P=0.052. In ProTaper group, percent of dentin removal at C level was slightly lower than in RevoS group, P=0.28. Strip perforation occurred in 8 samples and secondary fracture in 2 samples.\ Conclusions: The successful removal of fractured Ni-Ti instruments is more influenced by anatomical factors namely; the position of the instrument within the root canal, the location of the fractured instrument in relation to the root canal curvature and the remaining dentin thickness. Instrument's design indirectly affects success by requiring greater amount of dentin removal to disengage it.