Objective: To assess the effects of different class II cavities on stresses in amalgam and composite resin restorations. Material and Methods: Five kinds of class II MO cavities were prepared in the first molars of mandibles. The cavities were filled with amalgam and composite resin, respectively. Stresses were loaded on the teeth. Results: The stresses were greater on conventional cavities than modified cavities when 100N vertical and lateral pressures were loaded on the teeth, but the compressing stress and shear stress on cavities filled with amalgam were less on conventional cavities than on modified cavities. The stresses were significantly less on cavities filled with resin than those filled with amalgam when the cavities were designed with a slot-retention groove, without a slot-retention groove, and in plate form. The stresses were less in 2 kinds of slot and plate cavities than in conventional and modified cavities. Conclusion: It is better to design cavities with a slot-retention groove for amalgam restorations and without a slot-retention groove for composite resin restorations.