Aim: The aim of this study is to evaluate the effect of using different irrigating solutions (EDTA and Sodium hypochlorite (NaOCL)) during root canal treatment combined with different composite core materials (Multicore Flow (MCF) and SonicFill (SF) on the fracture resistance of endodontically treated teeth. Material and Methods: 40 recently extracted mandibular first premolar teeth were selected. Teeth were randomly divided into two groups (n=20) according to the irrigating solution used during root canal treatment; Group I: EDTA, Group II: Sodium hypochlorite (NaOCL). Each main group was further subdivided into two equal subgroups (10 each) according to core material used; Subgroup A: Multicore Flow (Ivoclar Vivadent, Schaan, Liechtenstein) (MCF), subgroup B: SonicFill (SF) (Kerr, Orange, CA, USA). The force in newton (N) was applied obliquely to the buccal cusp tips utilizing a specially designed attachment at cross head speed of 0.5mm/ minute until fracture and the maximum loads at which failure occurred were recorded. Results: no significant difference among the tested core material was recorded, however a significant difference with irrigating solutions was recorded where sodium hypochlorite significantly decreased fracture resistance more than EDTA with both core materials.