A new 28 GHz - 1 MW gyrotron has been developed for ECRH system of GAMMA 10 tandem mirror. To achieve the design target of the new gyrotron, which is 1 MW output for 1 s, the magnetron injection gun (MIG) and cavity were designed in detail with calculation codes. A first tube has been fabricated, and performance test is in progress. The maximum output power of 1.04 MW and the maximum output efficiency of 40.6% were obtained with short pulse operation. However, at the same time, it was confirmed that the output efficiency had decreased with the increase of beam current. This is explained with decrease, which is a one of issues to develop a higher efficiency gyrotron.