The plate characteristic functions are used to express the deflection shapes in the Rayleigh-Ritz method to study rectangular plate vibrations. Since the plate characteristic functions are reasonable approximations to the vibration modes, they are found to improve the convergence of vibration frequencies. These plate frequencies are used to check the accuracy of the Rayleigh frequencies associated with the plate characteristic function modes. Computations are carried out for each of the four mode categories of a clamped square plate. In each mode category, the first 30 natural frequencies are tabulated and the first nine mode diagrams are drawn. The results show that the choice of specific shape functions enhances the effectiveness of the Rayleigh-Ritz method.