From 2003 through 2004, our newest combined cycle plants, which applied "1500°C-class M501G gas turbine" started commercial operation in various region of United States. These plants are characterized not only by the achievement of high thermal plant efficiency with unique steam-cooled combustion systems, utilizing cooling steam from HRSG, but also lower air emissions, strictly limited by environmental standard and regulations. In this thesis, the "MYSTIC COMBINED CYCLE PLANT", which became the first Mitsubishi M501G unit in United States, is mainly introduced.