Vibration analysis of the plate is an important topic in high-speed train body design. Because of the dynamic loads on plates which are used in the wagon body of the train, vibration analysis and determination of the amount of deflection and bending of the structure is important in wagon design. A plate which is used in the high-speed train is composite plate. Composite plates are considered because of many advantages relative to the other plates, such as low weight, high strength and cost-effective. In this paper, the nonlinear free vibration analysis of the used plate in the wagon body of high-speed trains has been presented. First, a three layers sandwich plate used for car body of high-speed trains has been transformed into a single layer equivalent orthotropic plate. Von-Karman theory and the Galerkin method have been employed to solving the equations of motion of the equivalent orthotropic plate. The nonlinear natural frequencies of the first four modes of the system have been determined using the numerical and variational iteration methods (VIM). Then the effect of different parameters on the value of nonlinear frequencies of the first four modes has been studied. The Difference lower than 0.1% is observed between the determined natural frequencies by VIM, with initial condition limited to zero, and natural frequencies determined by linear vibration. The results show that natural frequency is increased by increasing elasticity modulus of the face, the thickness of the core and the thickness of the face of the sandwich plate. In addition, because of nonlinearity of plate vibration equations, natural frequencies of composite plate are increased by increasing initial condition.