In this research dynamic instability and nonlinear vibration of a clamped-clamped micro-beam sandwiched with piezoelectric layers based on parametric excitation in sub-harmonic region is investigated. The equation of motion is derived based on Hamiltonian principle, and non-dimensionalized using appropriate non-dimensional parameters. Applying a harmonic AC voltage to the piezoelectric layers results in the time varying of the linear stiffness of the micro-beam. The resultant motion equation in non-dimensional form is discretized to single degree of freedom model using Galerkin technique. The governing equation is a nonlinear Mathieu type ODE, and the periodic attractors are captured based on the shooting technique. The nonlinearity of governing equation is due to the geometric nonlinearity which originates from the clamped-clamped boundary conditions. The effect of various parameters including, magnitude of the nonlinear stiffness, damping coefficient, the frequency and the amplitude of the harmonic excitation on the parametric resonance region is investigated. The results depict that increased damping coefficient leads to the decreased aria of the parametric resonance region. It is concluded that the magnitude of the nonlinear stiffness, does not affect on the area of the resonance region, however it considerably influences on the amplitude of the parametric resonance.