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Objectives: Controversy exists regarding cavity preparation for restoration of in-terproximal caries in posterior teeth in terms of preserving the tooth structure and suitable stress distribution. This study aimed to assess the effect of extension and type of class II cavities and the remaining tooth structure in maxillary premolars restored with composite resin on the biomechanical properties of teeth using finite element method (FEM). Materials and Methods: Using FEM, eight three-dimensional (3D) models of class II cavities in maxillary premolars with variable mesiodistal (MD) dimen-sions, variable thickness of the residual wall in-between the mesial and distal cavi-ties and different locations of the wall were designed. Other dimensions were the same in all models. Cavities were restored with composite resin. A load equal to the masticatory force (200N) was applied to the teeth. Finite element analysis (FEA) was used to calculate the von Mises stress. Results: Stress in the enamel margin increased by increasing the MD dimension