Symmetric deformation analysis of sandwich plates with symmetric and balanced laminated faces and an orthotropic core is presented here. The transverse shear and normal strain and stress effects on symmetric deformation of such sandwich plates are considered. A set of twelfth-order governing equations and consistent boundary conditions are given from a mixed variational theorem. The symmetric deformation of square sandwich plates with fully simple supports subjected to symmetric sinusoidal loading on both surfaces is evaluated. The numerical results are compared with elasticity solutions. The present theory is found to agree approximately with three-dimensional elasticity solutions.