Considering uncertainties in realization of controllers, sufficient conditions under which the robust flexible controller exists are developed based on state feedback and in terms of the method of linear matrix inequality (LMI). A procedure for the design of flexible controller is presented. Different from the commonly used robust control method , in which the controller must be realized exactly, the controller gain obtained by the proposed method is at intervals. Therefore, when parameters of designed controllers vary in permitted ranges, stability and performance of the closed system can be assured, i.e. the designed controller is flexible, robust and reliable. A numerical example shows that the flexible controller designed by the presented method is effective, feasible and excellent in resisting the excursion of parameters of controller and suppressing the external disturbances.