The purpose that we did this study was to assess the adaptation of the margin, modes of fracture , and failure loads of composite crowns featuring different substructure on premolar teeth that had undergone root canal treatment. The study involved 48 lower teeth with only one root canals, which divided into equal six groups: a control group which received no treatment, a group that restored endodntically and had access cavity restored with composite resin, and four groups that underwent root canal treatment, had a ferrule (2mm), and received a standardized composite resin crown (SRCC) with different post materials (glass fiber, zirconium, or cast gold). Marginal adaptation was assessed by scanning the tooth-cementing composite (IF1) and cementing-composite-crown (IF2) interfaces using an electron microscope after all teeth were subjected to a thermocycling and mechanical load test in a computer-controlled and regulated masticator. The findings showed that following exposure to thermocycling and mechanical stress at IF1, marginal adaption declined markedly in groups III and IV, and in group V, at IF2. Half of the specimens showed signs of root fracture regardless of the substructure employed, and there was no statistically significant difference in the main loads to failure between the groups that received SRCCs. All of the examined posts had a good beneficial influence on adaptation of the margin at IF1, but none of them affected the failure modes or loads that caused failure of resin composite crowns.