Purpose: The purpose of this in vitro study was to evaluate and compare the fracture resistance and retention of CAD/CAM endocrowns and conventional glass fiber post supported crowns. Materials and Methods: Sixty-four (N=64) sound mandibular molars were endodontically treated and randomly assigned into 2 groups (n=32 each) according to the type of restoration constructed; Group (E): Endodontically treated teeth restored with IPS e.max CAD, lithium disilicate based, endocrowns and Group (P): Endodontically treated teeth restored with glass-fiber posts, composite cores and IPS e. max CAD crowns. Samples of each group were further subdivided into 2 subgroups (n=16 each) according to the type of adhesive resin cement used. the samples were thermocycled (2000 cycle, between 5 o C-55 o C). Samples were mounted in a universal testing machine and loaded to failure at a crosshead speed of 1.0 mm / min. The failure loads were recorded. Data were analyzed using one-way analysis of variance (ANOVA) and Tukey's post hoc significance difference tests. Differences were considered significant at P<0.05. The pull-out test was performed on a universal testing machine and the values obtained were statistically analyzed by analysis of variance using one-way analysis of variance (ANOVA)) and multiple comparison test of Tukey, with level of significance at P<0.05. Results: Statistical analysis using twoway analysis of variance (ANOVA) test revealed no statistically significant difference in failure load among the four tested subgroups (at P< 0.05). Endocrowns recorded statistically significant mean higher fracture load values (1729.91N±407.9) compared to post retained crowns, (1435.84±405.2). Statistical analysis using two ways analysis of variance (ANOVA) test revealed a statistically significant difference in debonding load among the four tested subgroups (at P<0.05). Tukey's post hoc test revealed a significant difference between each two subgroups. The highest mean debonding load was recorded in the subgroup (PR) (96.98N±4.47), whereas the least value was recorded in the subgroup (EM) (49.48N±3.81). Conclusions: lithium disilicate based endocrown restorations increase the fracture resistance of endodontically treated molars compared to conventional crowns associated with glass fiber posts and resin composite filling cores.