This paper investigates the free vibration of the FGM (Functionally graded materials) micro-beams under combined electrostatic, intermolecular forces and axial residual stress.The eigenvalue problem which describes the free vibration of the micro-beam at its statically deflected state is solved using DQ(differentiai quadrature) method.The natural frequencies of micro-beams for fixed-fixed boundary conditions are obtained.The comparison shows that the solutions are consistent with the experimental results reported in previous studies.A parametric study is conducted to show the significant effects of geometric nonlinearity, volume fraction index of Functionally graded materials, Casimir force, axial residual stress and ground electrode shape for the natural frequencies.