Objective: The aim of this in vitro study was to investigate the fracture strength and cuspal deflection of endodontically treated premolars restored using different composite resins along with or without fiber post application. Materials and Method: Eighty intact premolars were randomly divided into eight groups (n = 10); CO group: intact teeth (control), OPR group: mesio-occlusal-distal-palatal (MODP) preparation (OPR) + endodontic treatment (ET), TC group: MODP preparation + ET + Tetric N-Ceram, TB group: MODP preparation + ET + Tetric EvoCeram Bulk Fill, SO group: MODP preparation + ET + SonicFill 2, TC-P group: MODP preparation + ET + Hahnenkratt glass fiber post + Tetric N-Ceram, TB-P group: MODP preparation + ET + Hahnenkratt glass fiber + Tetric EvoCeram Bulk Fill, and SO-P Group: MODP preparation + ET + Hahnenkratt glass fiber post + SonicFill 2. After thermocycling, specimens were subjected to a compressive load until fracture. Data were analyzed using analysis of variance and Tukey tests (P < 0.05). Results: The mean fracture strength of groups which received post treatment showed similar fracture strength values [TC-P (931.6 97.9), TB-P (882.0 59.7), SO-P (862.0 143.0) (P 0.05)] and was significantly higher than OPR (530.6 41.7), TC (841.2 93.1), TB (774.5 101.8), and SO (735.0 178.01) groups (P < 0.05). No significant difference was detected among groups considering cuspal deflection (P 0.05). The fiber post insertion resulted in more unfavorable fractures. Conclusion: Endodontically treated teeth restored with fiber post and bulk-fill or conventional composite resins demonstrated fracture strength values similar to intact teeth.