Abstract. Development of 1 MW gyrotron is underway in JAERI. In previous gyrotron, a parasitic oscillation in the region between an electron gun and a cavity (beam tunnel) had occurred, that degraded the performance. The parasitic oscillation was suppressed by installing a silicon carbide on the beam tunnel wall. Consequently, the output of 1.3 MW was achieved at 170 GHz gyrotron, and the quasi-CW operation of 0.9 MW (for 9.2 s) and 0.5 MW (for 30 s) were obtained. It took almost no conditioning time to extend the pulse duration from 10 s to 30 s at 500 kW, which gives a promising view to the ITER CW gyrotron. The 110 GHz gyrotron that is designed based on the technology of the 170 GHz gyrotron attained the output of 1.2 MW for 4.1 s and 1 MW for 5 s. Four 110 GHz gyrotrons are applied to the EC system of JT-60U. A torus CVD diamond window of 31.75 mm in aperture has shown a capability of stable 1 MW transmission. The maximum injection performances were 3 MW for 2 s and 2.8 MW for 3.6 s. A long pulse injection of 1.5 MW for 5 s was also achieved. The EC system contributed to realize high electron temperature plasma of 26 keV.