The linear state-feedback control is one of the basic and fundamental part of the linear control theory. Eigenvectors are used through the singular value decomposition (svd) in robust analysis and synthesis of robust controllers in multi-input multi-output systems. In this paper the problem of robustness of a decoupling controller is analyzed. In particular, it is shown that the robustness of a decoupling controller is achieved using subspaces closer to the eigenvectors associated to the system. The analysis of the robustness is based on a perturbation method and a synthesis of a controller is shown. An application concerning an electrical motor is shown.