In this study, we consider the symmetric functionally graded material (FGM) for simply supported beam, which properties are changed functionally. The non-dimensional circular frequency with different properties and different shear function is determined. Hamilton’s principle is used to derive government equation, and boundary conditions. Separate the variables of displacement into position and time by using the method of separation. Then we simplify motion equation to an ordinary differential equation by using Galerkin’s method. The relationship between amplitude and time would be determined by using the 4th order Runge-Kutta method, The result reveal that n of P-FGM increases and the non-dimensional circular frequency also increases, p of S-FGM increases and the non-dimensional circular frequency decreases.