The "H" series gas turbine is a high efficiency and high power output gas turbine. The total thermal efficiency is about 2% higher and the output power is about 10% greater than that of a G series gas turbine. The reason the "H" series gas turbine can achieve a higher efficiency and a larger output is that its high temperature turbine firstand second-stage vane/blade are cooled by steam supplied from the bottoming cycle, instead of conventionally used air. Accordingly, the amount of bleed air from the compressor can be reduced. In 1999, the first trial operation of the "H" series gas turbine was conducted in the in-house verification power generation facility installed at the Takasago Machinery Works of Mitsubishi Heavy Industries, Ltd. (MHI). A second trial operation run was subsequent completed successfully at the same facility in 2001. Consequently, MHI has completed development of this new gas turbine using the prototype of the "H" series gas turbine. This paper reports on the development of the "H" series gas turbine.