A unified methodology of structural and/or control design for mechanical systems based on a polynomial matrix approach is proposed. A closed-loop D-stability specification is considered in this paper. By employing the polynomial matrix approach structural design parameters of mechanical systems, e.g., the mass, the damping and the stiffness and a feedback controller that is allowed to have a specified structure can be obtained simultaneously by solving LMIs. A simple iterative algorithm to obtain a better D-stable closed-loop system and a better central polynomial matrix is proposed. Design examples are provided to show the effectiveness of the present idea.