This study aimed to evaluate the influence of inlays/onlays and their material on stress distribution in mandibular molars with large cavities, using finite element analysis (FEA). Methods: 3D models of the first mandibular molar were created. Then, a mesio-occluso-distal cavity was created, and cusps were reduced (1.5 mm for buccal cusps and 1 mm for lingual cusps). The restorations were: inlay, onlay that covered buccal cusps (B models), and onlay that covered all cusps (LB models). Inlays and onlays were represented by two materials: nanofill composite resin and polymer-infiltrated ceramic network (PICN). Vertical load of 600 N was applied and von Mises stresses were calculated.