This paper presents a new methodology for diagonal dominance of large-scale systems via eigenstructure assignment. For a given large-scale system in general form, an equivalent descriptor system in the input-output decentralized form is defined. Sufficient conditions for diagonal dominance of the closed-loop system are introduced. These conditions are in terms of the isolated subsystems. Based on them, interactions between subsystems can be considered as external disturbances for each isolated subsystem. Then a previously proposed approach is used for disturbance attenuation via dynamical output compensators based on complete parametric eigenstructure assignment. By attenuating disturbances, closed-loop poles of the overall system are placed in a desirable region, by assigning the eigenstructure of each isolated subsystem appropriately. The presented algorithm alleviates the necessity of choosing a suitable frequency in designing a pre-compensator, as required by previous methods. The designed controller is in the decentralized form and plays the role of pre-compensator as well. An illustrative example is given to show the effectiveness of the proposed method.