This in-vitro study was designed to determine the influence of post adaptability on fracture resistance and fracture mode of endodontically treated teeth (ETT) with a flared root canal. Extracted human single-root mandibular premolars were selected according to inclusion criteria. A total of twenty-four teeth were selected. Teeth were assigned into 3 groups of 8 each. After endodontic treatment, experimental groups received restorations according to their restorative approaches: ETT restored without fiber post (Control group), ETT restored with prefabricated D.T. Light-Post (Prefabricated group), and ETT restored with D.T. Light-Post, relined with resin composite (Anatomic group). All specimens were loaded at 45 degrees in a universal testing machine until failure. The fracture resistance data were analyzed using one-way ANOVA and the fracture mode was assessed using the Chi-square test. Results showed that the anatomic group presented statistically significantly higher fracture resistance than the prefabricated group and Control group (p < 0.05). The prefabricated group showed no statistically significant difference from the control group regarding fracture resistance. For failure mode, there was no statistically significant difference among all groups. The control and prefabricated groups showed specimens with irreparable fracture mode. Therefore, an anatomically customized post is a reliable fabrication technique to restore flared root canal of ETT for better adaptation and improved fracture resistance.