The theoretical analysis of the problem of large amplitude vibration of thin elastic homogeneous plates has been treated by many authors [1][2][3]. Kung and Pao [4] have investigated the non-linear flexural vibrations of a thin clamped circular plate both experimentally and theoretically. Their investigation was restricted to the first axisymmetric approximate mode. They showed that there is close agreement between the theory and the experiments and they concluded that the theory and the approximate methods used in solving the non-linear differential equations are adequate for investigating the non-linear response of elastic plates. They also found a non-linearity of the hardening type when the amplitude of vibration is of the order of magnitude of the plate thickness. Theoretical derivations of the governing linear equations of sandwich beams and rectangular plates have been made by Kerwin [5], Ross et al. [6], Yu [7], Mead [8-10]. Yan and Dowell [11], Mead and Markus [12, 13], DiTaranto [14, 15], and Rao and Nakra [16,17]. An analysis of the linear vibration of layered circular plates was done by Venkatesan and Kunukkasseril [18]. Also, Markus and Nanasi studied the effect of the in-plane inertia force for three-layered circular plates [19]. In this note, the effect of the sandwich construction on the hardening type non-linearity for large amplitude forced vibrations of a thin clamped circular plate under a symmetrically distributed load, is presented. The equations of non-linear vibratory motion of the thin clamped circular unsymmetrical sandwich plate are derived, with its deflection being assumed in a polynomial form which satisfies the boundary conditions for a circular plate clamped along its edge. The following assumptions are made: (1) the longitudinal and rotatory inertia of the faceplates and core are negligible; (2) the faceplates are elastic and isotropic and suffer no shear deformation normal to the plate surfaces; (3) the core carries shear, but no direct stress (neither radial nor circumferential normal stresses) and is of linear viscoelastic material; (4) the influence of the shear in the core on the in-plane equilibrium can be neglected (5) there is perfect bonding between the faceplates and the core layer (no slip, no overlap and no gap across the interfaces).