Abstract: The generalized equations of motion is developed for high-speed planar elastic mechanism systems by the application of the finite dynamic element theory. The dynamic shape functions of planar frame element which include the influences of shear deformation and the moment of inertia are presented. An efficient closed-form iterative method for the solution of the equation of motion is described in this paper. The convergence condition is discussed. At last, a numerical simulation is carried out. Numerical results of an experimental mechanism presented herein indicate that the simulation solution obtained by the presented analysis method agrees significantly better with the experimental data than that obtained by the conventional finite element method.