A new dynamic model of compliant mechanisms is developed based on the pseudo-rigid-body model. The kinetic energy and potential energy of various kinds of compliant segments are derived using numerical methods at first. The dynamic equation of planar compliant mechanisms is then developed based on the Lagrange equation. The natural frequency is obtained in the example of a planar compliant parallel-guiding mechanism. The numerical results show the advantage of the proposed method for the dynamic analysis of compliant mechanisms.