The demand for high power millimeter and Terahertz waves according to the need in the field of nuclear fusion is increasing. Vacuum devices are thought to be the only solution for providing high power (> 1 MW) in the frequency range greater than 100 GHz for heating and current driving of fusion plasmas. It has been forcasted that approximately 30 MW with greater than 200 GHz frequency sources are required for the EC system for the future K-DEMO. Considering the requirement of the EC gyrotron, it is urgent to develop a gyrotron source with the frequency greater than 200 GHz. In this talk, I will present the recent effort to develop the high frequency gyrotron source and some activities in developing a compact vacuum electronics based high power source in Korea.