Aim of the study: The aim of this study was to evaluate the fracture resistance of endodontically treated teeth that were prosthetically restored with two fiber post designs (smooth and serrated) with different lengths (6,8and10mm). Material and methods: A total of Thirty intact upper central incisors were collected and decoronated 2mm coronally. Teeth were prepared 2mm ferrule circumferential and chamfer finish line of 0.5 mm. The teeth were embedded in epoxy resin blocks using a centralizing device. Root canals were prepared and obturated then post holes were prepared with the corresponding drill using the drilling machine, In this study the thirty samples were divided into 2 groups (n=30) according to the post surface design Group І : parallel smooth glass fiber post. (n=15).Group ІІ: parallel serrated glass fiber post. (n=15).The samples of each group were subdivided into three sub groups according to different post Lengths. Sub Group 1: lengths 6mm Sub Group 2: lengths 8mm.Sub Group 3: lengths 10mm..All posts were cemented with a-self-adhesive resin cement using a load applicator device. Full metal coping crown was constructed and cemented .Each the specimens were then subjected to a compressive test in a universal testing machine were load was applied on palatal side at angle of 135 degree in relation to long axis of the root at across head speed of 1mm/min until failure occurred . Results: There was no statistically significant difference between the tested post designs, and there was a statistically significant difference between the three post lengths. Conclusion: Different glass fiber post length and surface design would influence the fracture resistance of endodontically treated maxillary central incisor teeth. The use of short glass fiber post should be avoided as may lead to non-favorable fracture.