This paper proposes an optimum design for the dc-based DFIG generation system in a dc grid to make its cost optimal. As the stator is connected to a dc bus by the stator side converter, its voltage and frequency are not enslaved to an ac grid, which makes it possible to decrease the synchronous speed, rotor speed, and gearbox ratio. Three aspects are improved. First, control targets are optimized to minimize the increased generator current with decreasing gearbox ratio. Second, optimum design of a generation system is made by regulating gearbox ratio at the cost of increase in the generator current. Significant decreases in cost and losses are analytically obtained from the elaborated cost and losses models, whose parameters are taken directly from the practical and commercial wind generation system. Third, the coordinated model predictive control is adopted in the stator flux-oriented frame with a stator flux and rotor current predictive method. Finally, as the synchronous speed and rotor speed decrease to low levels, simulation and experiment studies are carried out to verify the operational feasibility of DFIG and analyze its steady and transient performance.