Primary parametric resonance of a parametrically excited simply supported thin rectangular plate on nonlinear elastic foundation is analyzed. The nonlinear vibration equation that has the form of a Mathieu-Duffing’s equation is derived from the Von Karman equation and Galerkin’s method. The method of multiple scales is used to find the averaged equation. Numerical results are presented to illustrate the effect of various parameters including damping parameter

foundation coefficient as well as geometry parameter.