A high power gyrotron is designed and tested. The design is based on the TE31,8 1 MW gyrotron. The higher power design is realized by choosing TE31,12 higher order oscillation mode. A basic test of the high power gyrotron is performed in JAEA test stand with short pulse and long pulse. In the short pulse experiments, 1020 kW output is confirmed with expected output beam pattern. In the long pulse experiments, it is successfully achieved stable 1 MW operation (1030 kW/2 s). The longest shot is 550 kW/~200 s. These results show successful design and fabrication of the TE31,12 high power gyrotron.