Abstract: The gyrotron is one of the most promising high-power millimeter-wave sources for electron cyclotron resonance heating (ECRH) in controlled thermal nuclear fusion experiments. In this paper, the design of a high-frequency interaction cavity of a 1 MW/140 GHz gyrotron is described in detail. The cavity is designed by using eigen mode analysis and radio frequency (RF) behavior calculation. Rounded transitions at the input and output tapers are designed for reducing mode conversion. With the obtained cavity structure, non-linear self-consistent equations are adopted to calculate its output power and efficiency. A particle-in-cell (PIC) method is used to simulate the beam-wave interaction process for obtaining the resonant frequency and output power of the cavity. The PIC simulation results match considerably well with the results obtained by the non-linear self-consistent calculation. The cavity is currently under construction and will be integrated with other components for overall testing.