The effect of structural optimization on optimal control design is studied in this paper. Structural optimiza- tion was treated as a problem of mass minimization with constraint on the open-loop frequency. The quadratic performance index, involving the state and control variables, was used in the design of the control system. A control system with only full-state feedback was considered. A procedure for generating the state and control weighting matrices by structural dynamics programs was outlined. By introducing simple scaling parameters, the weighting matrices were used effectively to achieve the desired control objectives. A number of case studies using a simple truss structure were made, in which vibration suppression with only initial disturbances was considered. The conclusion was that modification of the structural parameters (stiffness and structural mass) did not significantly alter the control design in this study. IBRATION control is an important consideration in the design of dynamic systems on the ground, in the air, and in space. The disturbances in ground and air vehicles are primarily caused by rough road (runway) profiles and airflow, such as gusts and powerplants. Similarly, in large space structures the disturbances are the result of slew- ing/pointing maneuvers, thermal transients, and mechanical machinery such as coolers, generators, etc. Control of the dynamic response is essential for maintaining the ride quality and performance requirements, as well as for the safety of the structure. The response of a structure is basically governed by three sets of parameters. The mass, damping, and stiffness repre- sent the structural parameters. The second set of parameters is due to the sources of external disturbances. These are generally external to the system and are considered as fixed inputs; thus their alteration is not within the realm of the structures/controls designer. The third set represents the control system, assuming that the structure is actively con- trolled. Control of the dynamic response by modification of the structural parameters alone is considered to be passive. Passive control is most appealing from both the reliability and maintainability points of view, if it can be achieved at all economically. Basically, the stiffness and mass modifica- tions result in frequency and mode changes, while the damp- ing affects the dissipation energy of the system. The damping can be significantly altered by either viscoelastic coatings (or constrained layer damping) or the provision of discrete dashpot mechanisms. The objective of vibration control is to design the structure and its controls either to eliminate vibration completely or to reduce the mean square response of the system to a desired level within a reasonable span of time. In addition, it is im- portant that this objective be achieved in some optimal way. For a structural designer, the optimal design represents an