The problem of designing decentralized optimal controllers for the load frequency control (LFC) of interconnected power systems is considered. An especially attractive feature of the proposed scheme is that it implements current practice in the area of LFC and current optimal control techniques to design the system supplementary controllers. The control law is assumed to have a proportional and/or integral form and is a function only of the measurable states (present-day-practice). The LFC problem is formulated in a parameter optimization form. By employing Pontryagin minimum principle, the optimal control parameters are obtained by solving a nonlinear two-point-boundary-value problem (present-day control theory). The values obtained for these parameters do not depend on the strength of the disturbance.