Aim : to compare the fracture resistance of weakened root structure using three different reinforcement materials namely: Mineral trioxide aggregate(MTA), grey Portland cement (GPC) and glass ionomer restorative material (GI) with and without the use of glass fiber post.Materials and Methods: Eighty human maxillary central incisors were collected, sixty specimens were randomly assigned into three experimental groups (n=20) according to the reinforcement material used. Group I: The roots were reinforced using mineral trioxide aggregate (MTA). Group II : The roots were reinforced using Grey Portland cement (GPC). Group III: The roots were reinforced using glass ionomer restorative material (GI). Each experimental group was further subdivided into two subgroups (n=10) according to whether the post was used or not. The remaining 20 samples were divided into two control groups (n=10):positive and negative control groups. A universal fracture resistance testing machine was used in this study, data was collected, tabulated and statistically analyzed.Results: The highest fracture resistance value was recorded by glass ionomer restorative material followed by MTA and Portland cement, while fracture resistance of MTA was similar to that of Portland cement with no statistical significant difference between both .All investigated groups without post showed high results than with post application.Conclusion: The usage of fiber post has weakened the tooth structure rather than reinforcing it, none of the reinforcing materials could reach the fracture resistance of dentin.