Aim: This study investigated the fracture resistance of different intrapulpal extensions of endocrowns made of lithium disilicate (L) and polyetheretherketone (P). Materials and methods: sixty extracted human mandibular first and second molars were split equally into three groups based on the depth of the intrapulpal cavities (0 mm, 2 mm, and 4 mm), and then each group was further separated into two subgroups according to the material of the endocrown (L and P). Fracture resistence was assessed by a computer-controlled testing machine, and then data were collected and analyzed using a two-way ANOVA followed by Tukey's post hoc test. Results: For endocrowns fabricated from polyetheretherketone, the fracture resistance values ranged from (2302±101.97 to 3363.22±382.32), which was significantly higher than that of lithium disilicate (1747.16±154.38 to 2756.73±126). The lowest values of fracture resistance for L and P restorations were 1747.16+154 and 2302.92+101, respectively, at 0 mm. depth designs, whereas the values increased to reach their maximum values of 2765.73+126 and 3363.22+382, respectively, at 4 mm depth designs. Conclusion: The fracture resistance of restorations made of P polyetheretherketone is higher than that of restorations made of lithium disilicate. Increasing intrapulpal cavity depths increases the fracture resistance of endocrowned restoration.