The HVDC and excitation decentralized controllers for parallel AC-DC system based on effective relative gain array and mixed H2/H∞ control are proposed. The results of effective relative gain array (ERGA) which combines the advantage of both relative gain array and dynamic relative gain array studies suggest the effect of control loop configurations. Based on the ERGA analysis and multi objective mixed H2/H∞ control synthesis, the designed controllers which simultaneously consider the output disturbance rejection, ensure the robustness against model uncertainties, and reduce the control cost could provide efficient damping against the inter-area and local modes of oscillations. The effectiveness of the new design is demonstrated with EMTDC/PSCAD simulation.