Nowadays, the power industry is in transition to a fully competitive scenario. Under this situation, some power system controls such as frequency control will be served as ancillary services. Then, it is important to consider how the system frequency should be controlled. In this paper, a robust frequency stabilization controller design method of solid-state phase shifter is proposed. The frequency controller of phase shifter located in series with a tie line between two areas, is designed based on the idea of enhancing the damping of inter-area oscillation mode. The method of Overlapping Decompositions for system reduction and the Η_∞ control via Normalized Coprime Factorization (NCP) approach are used to achieve the proposed design idea. By NCF approach, the weighting selection in Η_∞ control design is simplified. Simulation results confirm the performance and robustness of the proposed control against load disturbance and system uncertainties.