Statement of problem:Although endocrowns have proven their clinical success in many situations, the intra-chamber endocrown extension and the proportion between the clinical crown height and the radicular stump and their effects on their fracture resistance and mode of failure have not yet been verified.Purpose: Assess the effect of intra-chamber extension of lithium disilicate endocrowns on the fracture resistance and failure modes in endodontically treated molars after thermo-mechanical loading.Material and Methods: Forty extracted molar teeth were selected and divided into 5 groups (n=8) as follows: unprepared natural teeth (NAT; control); post and core supported crowns (PCC); teeth with 4 mm intra-chamber extension (EC4); 2 mm intra-chamber extension (EC2); 1 mm intra-chamber extension (EC1). Endocrowns were subjected to 5000 thermal cycles then 1,200,000 chewing cycles. Surviving specimens were vertically loaded in a universal testing machine until fracture. Data were analyzed by 1-way ANOVA, followed by pair-wise comparaisons with the Bonferroni post-hoc test (p ≤ 0.05). The mode of failure was determined by visual inspection.Results: Natural teeth had statistically significantly lower fracture resistance than groups with crowns (p= 0.002). However, there were no statistically significant differences between the crown groups with mean fracture resistance ranging from 2502.5 N to 2843.8 N. Endocrowns exhibited mainly unfavorable fracture failure. The amount of teeth destruction related to the group EC4 was less than that related to the group EC1. Conclusions:Endocrowns with greater intra-chamber extension provided insignificantly higher fracture resistance but more protection of underlying tooth structure than endocrowns with lesser extension.