The preservation of tooth structure during cavity preparation is crucial for maintaining tooth strength and longevity of restorations. The biomechanical behavior of teeth, especially those with mesio-occlusal-distal (MOD) cavity preparations, are significantly affected by the extent of cavity preparation and the type of restorative treatment employed. The aim of this study was to evaluate and analyze the stress distribution when a non-vital mandibular molar with MOD cavity is restored with Ribbond and direct composite filling, using finite element analysis.