With the development of MEMS fabricating technology, MEMS structure now has a wide application in manufacturing industry. Meanwhile, because of no any characteristic length scale parameters in constitutive relationship, the classical theory of continuum mechanics is unlikely to predict the size effect under micro scale. This paper investigates the nonlinear problems of micro Mindlin plates based on the firstorder shear deformation theory and von Karman nonlinearity, and the strain gradient theory is also utilized to take size effect into consideration. Nonlinear dynamic model of the micro Mindlin plate along with its boundary conditions are obtained by taking strain energy, including strain energy of both macro and micro deformation, kinetic energy and external work into Hamilton′s principle.