Endodontically treated teeth are usually weak and need prosthodontic restorations. The selection and design of endodontic posts are associated with the biomechanical behavior of teeth. This study analyzed the stress in endodontically treated teeth using the finite element method. The model of a restored tooth consists of the post, core, crown, dentin, periodontal ligament, cortical bone, cancellous bone, and gingiva. The post is given various design parameters: three diameters (1.3, 1.8, and 2.3 mm), four lengths (9.7, 12.7, 15.7, and 18.7 mm), three shapes (cylindrical, tapered, and two-step cylindrical), and five materials (gold alloy, stainless steel, titanium, carbon fiber, and glass fiber). Static loads of 100 N are applied in the vertical, horizontal, and oblique directions on the incisal edge, and the generated von Mises stress is calculated. Results show that increases in the post diameter and elastic modulus decrease the post stress. The dentin stress slightly decreases with increasing post stress. The loading mode greatly affects the peak dentin stress magnitude (horizontal force (37 MPa) > oblique force (28 MPa) > vertical force (9 MPa)) and location. Furthermore, the horizontal force induces harmful bending of the tooth structure.