This paper proposes a novel design criterion for manipulators with flexible joints based on elastodynamic performance. Consequently, the elastodynamic performance is assessed; the eigenvalue problem is solved to determine the first natural frequency based on the manipulator's inertia and stiffness matrices. Then, the elastodynamic criterion is evaluated within a region of the Cartesian workspace in order to obtain a global measure. The global elastodynamic performance of a planar serial manipulator and a planar parallel manipulator with flexible joints were obtained. The numerical results show a high dependency of the elastodynamic performance on the configuration and geometric parameters of the manipulator.