Use of control systems performance limitation is made to design mechanical structure of a counterbalance in a one-link arm that swings vertically forced by a direct-drive motor. It is well known that, by Bode's integral, sensitivity performance of the control systems is essentially limited and that the bandwidth of the open loop transfer function causes a kind of waterbed effects. The performance bounds are proved to be affected significantly by the poles location, the inertia moment, as well as the dominant characteristic frequency of the plant. Therefore it is claimed that a sensitivity specification provides useful designing conditions in terms of a plant, and that the plant with no unstable poles, a low inertia moment and a low characteristic frequency is generally desirable to meet it. The conditions are straightforwardly recited into ones for physical parameters such as mass, length and angle of the counterbalance. Effectiveness of the proposed approach for structure design of a counterbalance is illustrated by a standard controller synthesis and nonlinear numerical simulation.