AbstractRF heating system with total power of 24 MW fed by 1 MW klystrons at around 2 GHz have been developed and operated successfully in the JT-60 tokamak. System performance has been demonstrated in a real plasma load. A super high power system has worked stably against changeable plasma load without output circulators. A maximum power input into plasmas reached 11 MW from three array launchers of four row by eight line waveguides, which corresponded to 17 MW generator power.