The purpose of this in vitro study was to compare the stress distribution and natural frequency of different shape and thickness retainer designs for maxillary posterior resin-bonded prostheses using finite element (FE) method. A 3D FE model of a three unit posterior resin-bonded prosthesis analysis model was generated. Three different shaped retainer designs, viz. C-shaped (three axial surface wraparounds), D-shaped (three axial surface wraparounds with central groove) and O-shaped (360° wraparounds), and three different thicknesses, viz., 0.4, 0.8, and 1.2 mm, resin-bonded prostheses were used in this study. The resin-bonded prosthesis analysis model was imported into an FE analysis software (ANSYS 10.0, ANSYS, USA) and attribution of material properties. The nodes at the bottom surface of the roots were assigned fixed zero displacement in the three spatial dimensions. A simulated angle of 45° loading of a 100 N force was applied to the node of the pontic lingual cusp surface. The stress distributions and corresponding natural frequencies were analyzed and resolved. The C-shaped retainer for 0.4 mm thickness recorded the greatest von Mises stresses of 71.4 MPa for all three groups. C-shaped, D-shaped and O-shaped retainer presented natural frequencies 3,988, 7,754, and 10,494 Hz, respectively. D-shaped retainer and O-shaped retainer increased natural frequencies and structural rigidity over the traditional C-shaped retainer. The maximum von Mises stresses values of the remaining tooth and prosthesis decreased with greater retainer thickness. D-shaped retainer and O-shaped retainer increased natural frequencies and structural rigidity over the traditional C-shaped retainer.