Objective: The objective of this study is to evaluate the fracture resistance and failure mode of endodontically treated teeth restored with lithium disilicate endocrowns of different crown thicknesses. Materials and Methods:A total of 30 endodontically treated permanent mandibular first molars were sectioned horizontally 2 mm above the highest point of the cemento-enamel junction. The specimens were divided into three groups, and each group was restored with lithium disilicate (IPS e-max press) endocrowns of different crown thicknesses (3, 4.5, and 6 mm, respectively). After cementation, specimens were stored in room temperature for 72 hours, followed by subjecting them to compressive strength testing until failure. The fracture loads and the failure mode were recorded. Statistically analysis was performed using one-way analysis of variance.Results: A statistically significant difference was found in the fracture resistance between the three groups with the highest fracture resistance in the 3 mm group, followed by the 4.5 mm group, and the least in the 6 mm group (P < .05). Most of the failures were accompanied with tooth fracture (90% in 3 mm group, 100% in 4.5 mm group, and 80% in 6 mm group). Conclusions:Increasing the crown thickness of the endocrowns reduced the fracture resistance of restored teeth. Minimum fracture loads for all teeth restored with endocrowns were significantly higher than the maximum occlusal forces reported in the literature.Clinical Significance: Fracture resistance and mode of failure of lithium disilicate (IPS e-max press) endocrowns varies widely between crown thicknesses. Clinicians should be cautious with crown thickness for endocrown restorations. K E Y W O R D S endocrowns, endodontically treated teeth, failure mode, fracture resistance, lithium disilicate