Background: Endodontic treated teeth present a higher fracture rate. The fracture strength of endodontically treated teeth is affected by access cavity design and cervical root canal dentin thickness. Aim: To compare the effect of access cavity design and two different root canal tapers in extracted mandibular premolars on fracture strength of endodontic treated teeth and remaining cervical dentin thickness. Materials and Methods: total of (20) lower premolars were selected and divided into 4 groups. In group (A) five teeth were accessed traditionally and prepared by CMA rotary system, in group (B) five teeth were accessed traditionally and prepared by M3 Pro Gold rotary system. In group (C) five teeth were accessed conservatively and prepared by CMA rotary system and in group (D), five teeth were accessed conservatively and prepared by m3 pro gold rotary system. Remaining dentin thickness through CBCT scanning and Fracture strength was measured through universal testing machine. Results: No significant difference between the removed dentin thickness of treated teeth either with M3 pro gold or CMA at coronal and middle levels however at the apical level, CMA showed statistically significantly lower median remaining dentin thickness than M3 Pro Gold. No significant difference between fracture resistances of treated teeth either with traditional or conservative access.Conclusion: Conservative design failed to intensify the fracture resistance of mandibular premolars. Conservative cervical taper within 4% and 6% presented similar effect on cervical dentin thickness in mandibular premolars.