For answering such a question further details should be furnished. Just for instance
1. What kind of grading rule is adopted?
2. Correspondingly, what kind of constitutive response result?
3. When you refer to natural frequency, what is the mode you refer?
In detail, depending on these aspects, it could be possible that the grading rule is such that, with reference to the selected vibration mode, it does not affect the corresponding natural frequency in comparison with the isotopic case.
i have adopted rule of mixture gradation in the form of power law. i have analysed my problem in ansys and analytically solved by Rayleigh Ritz method through matlab coding. then after i found that the natural frequency of isotropic and functionally graded square plate is same for CCCC boundary condition.