Objective: The aim of this study was to evaluate the marginal adaptation and fracture resistance of endocrowns made from Lithium disilicate (Emax CAD) and CAD/CAM composite blocks (Brillient Crios) restoring mandibular premolars. Materials and Methods: Fourteen extracted human single rooted mandibular premolars were endodontically prepared in a standardized way to receive CAD/CAM fabricated endocrowns, the samples were divided into two groups (n=7); E. max CAD (EM) group and Brillient Crios (BC) group. Marginal gaps (µm) evaluated under stereomicroscope after cementation and repeated after thermocycling. Fracture load was recorded and Failure modes were determined by digital microscope and scanning electron microscope SEM. Results: EM group verified higher vertical marginal gap mean value than BC either before or after thermocycling, the difference was statistically significant as indicated by two-way ANOVA (p>0.05). EM group recorded significantly higher fracture resistance mean values (737.55±223.3N) than BC (434.33±100.8 N) group, (p < 0.05) as verified by unpaired t-test. The failure mode presented that (71.44%) of BC specimens exhibited favorable type and (28.57%) catastrophic failure. Whereas, EM group revealed (71.43%) catastrophic failure and (28.57%) favorable type. Chi square test (P < 0.05) displayed statistically significant difference between both groups. Conclusions: Brilliant Crios endocrowns showed better marginal adaptation and more favorable failure mode than those fabricated from E max CAD. E max endocrowns revealed higher fracture resistance than Brillient Crios.