Purpose: Two commonly used attachment systems for implant-retained overdentures are bar and ball systems. The aim of this study was to compare these two types of attachments on the basis of the stresses introduced to the mandibular posterior residual ridge by an overdenture retained by two implants. Method and Materials: A basic model was generated from the data provided by a CT scan of a patient's mandible, the diagnostic casts, and the existing overdenture. Two root-form implants were placed in the anterior mandible. Models were used to simulate two situations: The first model represented an edentulous mandible supporting an overdenture retained by two implants connected with a bar and plastic clip system, and in the second model two ball attachments with nylon caps were used as the retention mechanism. A 35 N vertical load was introduced to the first molar region of each group to simulate the occlusal forces of the maxillary complete denture. The stresses in the mandibular residual ridges under the first molar region were measured using Ansys software. Results: The bar-retained overdenture introduced higher stresses to the posterior mandibular residual ridge compared to the ball-retained overdenture (0.4 MPa and 0.1 MPa, respectively). Conclusion: Within the limitations of this study, the ball system was shown to introduce a lower amount of stress to the posterior mandibular residual ridge compared to a bar and clip system. Keywords: ball attachment, bar attachment, finite element analysis, mandibular implant-retained overdenture, posterior residual ridge