Aim: This study aimed to investigate the stress patterns in weakened teeth restored with three different types of post core restorations under a vertical load using the finite element method. Methods: Four axisymmetric models were created by computer software to simulate the lower second permanent premolar in different situations. Situation 1: natural unrestored tooth, 2: weakened tooth restored with fiber reinforced composite post and core, 3: weakened tooth restored with gold alloy post and core, and 4: weakened tooth restored with zirconia post and core. The models included a post and core without a crown, gutta percha, resin cement and supporting structures. Models were considered isotropic, and exhibiting complete adhesion. A vertical static load of 40N/mm² was applied to the occlusal surface. Von Mises tensile stresses were calculated in the model and presented in the form of a colored stress map. Results: Stress patterns in the fiber reinforced composite were closest to the stress pattern of natural tooth with most stresses found in the coronal area dissipating as they travel apically. Zirconia and gold post and cores had more stress concentration areas within the post and more apical than natural tooth or fiber reinforced composite. All post systems showed an area of stress concentration in dentin around the end of the post especially the zirconia post and core, however they were within the range of dentin tensile strength. Conclusion: zirconia posts are more likely to cause fracture in the root in weakened teeth when compared to fiber reinforced composite post and core. Fiber reinforced composite exhibits the closest biomechanical behavior to normal teeth.