Abstract: Objective To evaluate the effect of different endodontics cavity accesses on stress distribution of endodontics treated mandibular first molar with a three dimension finite element analysis Methods With Micro-CT scans of an intact mandibular first molar，four finite element analysis models of mandibular first molar are designed and constructed. An intact teeth model (group A)，a contracted endodontic cavity model (group B), a model traditional endodontic cavity without straight way established (group C),a traditional endodontic cavity model with straight way established (group D). Endodontics treated Models were filled with gutta pertcha and then restored with resin. All models were subjected with four different forces. The stress distribution patterns and the maximum Von Mises (VM) stresses were calculated and compared. Results The peak VM stress of vertical force: C > D > B > A, the peak VM stress of horizontal force: B > A> C > D , the peak VM stress of 45 °force: D > B > C > A, the peak VM stress of the largest bite force: C > D > A > B. Except the stress loading points, the stress concentration area is mainly located in PCD. Conclusion With the limitation of this study，the resistance to fracture of endodontics treated mandibular first molar with CECs is better when vertical force was submitted. To preserve more PCD is positive to improve the resistance to fracture of endodontics treated mandibular first molar.

Key words: minimally invasive endodontics, first mandibular molar, biomechanics, finite element analysis