In designing a wide-area controller for a power system， a problem not to be neglected is the transmission time-delay. The free-weighting matrix method based on linear matrix inequality（LMI） is used to stabilize power systems with time-delay. And a memoryless state feed-back controller is then proposed. Systems such as the WSCC 3-generator 9-bus system is employed to validate the effectiveness of the method presented， which is compared with some existing approaches. It has been found that the method given is even more effective and less conservative in helping with wide-area controller designing.