This paper introduce the real time digital simulator which is located in Korea Electric Power Research Institute. This paper also describes the methodology for the performance test of the PSS using KEPS. This test is to get a high degree of the confidence of the developed PSS before it is installed into the real power system. This has been performed in the form of closed-loop tests in which Simulator and PSS are connected and signals come and back interactively. Many tests have successfully done using KEPS which consists of 26 RTDS racks, under the large-scale power system. The simulated reduced KEPCO power system contains 88 generators and 295 buses. Through the AVR step, three phase fault and active power variation test, the effectiveness of developed PSS has been proved. This paper also presents the overview of KEPS and hardware of protype PSS.