To meet the demands of increasing electrical power supply, substations must be constructed underground in urban areas. For underground substation equipment, oilless equipment is preferable and therefore a gas-insulated power transformer was developed. The separate-cooling/sheet-winding design concept is employed. In 1989, a 154 kV-200 MVA gas-insulated power transformer was commissioned and has been operated with satisfactory operating records. Utilizing this development experience and introducing new technology, a 275 kV-300 MVA gas-insulated power transformer has been developed. The compact design of the main-gap insulation is very important and its reliability has been verified. The turn-to-turn insulation, which is made of PET film insulation, has also been verified under severely-stressed test conditions. The design of the 300 MVA gas-insulated transformer is described.< >