The purpose of this study was to compare the fracture strength and fracture pattern of a previously endodontically treated premolar with MOD cavity when restored with a resin composite together with prefabricated fiber-reinforced composite (FRC) and novel unpolymerized fiber-reinforced composite (UPF) posts. Forty intact human maxillary premolars with single root and two canals were embedded in resin molds with simulated periodontal ligament. The specimens were divided into five groups: 1) Sound premolar (positive control); 2) Non-restored endodontically treated premolar with MOD cavity (negative control); 3) Endodontically treated premolar with MOD cavity restored with resin composite; 4) Endodontically treated premolar with MOD cavity restored with FRC post and resin composite; 5) Endodontically treated premolar with MOD cavity restored with UPF post and resin composite. All specimens were subjected to 500,000 cycles of cyclic loading and 10,000 cycles of thermocycling. The specimens were loaded to fracture at an angle of 45° on palatal cusp. The sound premolar had the highest fracture strength (510.92 ± 106.54 N) while the non-restored premolar had the lowest strength (73.88 ± 20.52 N). Using the post with resin composite restoration significantly increased the strength of the tooth. However, there was no significant difference in the strength between using FRC and UPF post. Most of the specimens had a favorable fracture. In conclusion, fiber-reinforced composite post positively increased the fracture strength when restored endodontically treated premolar with MOD cavity using resin composite but did not affect the fracture pattern. The type of post did not affect the fracture strength of the restored tooth.