This paper develops a methodology for the optimal design of reliable control system. For a class of large-scale systems, optimal reliable guaranteed cost stabilization with continual gain actuator faults is presented. Based on the lyapunov method and linear matrix inequality, a sufficient condition for the existence of decentralized dynamic output feedback reliable controller is given. The design guarantees the system asymptotic stability and minimizes the upper bound of a given quadratic cost function. Simulation results show the effectiveness of the design method.