Aim of the study: This in vitro study was conducted to investigate the fracture strength and microleakage of simulated immature teeth reinforced with fiber post using two types of composite resin cement “ total etch and self etch “. Materials and methods: Eighty freshly extracted maxillary central incisors were used in the study, they were divided into two groups (40 samples for each) according to the stage of root development (mature – immature), Then each group was subdivided into two subgroups (20 samples for each) according to the type of adhesive composite resin cement (RELY X TM U200 Automix) (3M ESPE Germany) self-adhesive and (Bifix QM) ( Voco. GERMANY) total etch , Each subgroup was further divided into two classes (10 samples for each) according to subjecting the samples to thermocycling or not , Finally 5 samples from each class were tested for fracture strength by using universal testing machine and the other 5 samples of each class were tested for microleakage by using digital microscope. Results: The results of the fracture strength test showed that mature teeth showed higher fracture strength than simulated immature teeth. Total etch composite resin composite cement showed higher fracture strength than self etch cement. Finally the results showed that thermocycling affects negatively on fracture strength. As regard microleakage, the reults pointed out that no microleakage occurred in all tested groups. Conclusions: Fracture strength of mature teeth is more than immature teeth, using total etch composite resin cement give more fracture strength than self etch composite resin cement, thermocycling affects negatively the fracture strength for mature or immature endodontically treated teeth, no microleakage occurred irrespective to the state of restored root (mature or immature).