Abstract: A novel electrostatic harmonic micro-motor is proposed and its operating principle is described. The necessity to investigate out-plate dynamics for the drive system is analyzed. Based on deformation energy, kinetic energy and electric field energy, the dynamic differential equation is obtained by means of Hamilton principle. The equation is solved for two different modes and the equation of the natural frequency is derived. The changes of the natural frequency along with the system parameters are given. The results are useful for the dynamic characteristic design of the micro-motor and the reasonable system parameters determination.