Abstract: It is one of significant ways that magnetic confinement fusion energy can fundamentally solve human energy problems which human beings have realized. And the electron cyclotron resonance heating (ECRH) and electron cyclotron current drive (ECCD) are the main heating method to keep the temperature of the fusion plasma up to approximate 100 million degrees which is an important condition for stable operation of nuclear fusion. The gyrotrons are the key electronics of high-power microwave sources for ECRH system. In this paper, the principle, worldwide progress and future development of mega-watt level gyrotrons are presented.