Secondary Surveillance Radar (SSR) is an important aircraft surveillance system, which provides aircraft's position, altitude and identification. However, because of the increase of air traffic in recent years, many ploblems, such as interference, insufficient accuracy, limitation of data handling, have occurred. In order to solve these problems, International Civil Aviation Organization (ICAO) has adopted the international standards of SSR mode S which has improved surveillance and air-ground datalink. Electronic Navigation Research Institute (ENRI) has developed SSR mode S experimental system and conducted experiments on surveillance. The results shows that ENRI's SSR mode S satisfies the surveillance criteria of ICAO. This paper describes development of SSR mode S and the results of the experiments.