Abstract: In this paper, a high efficiency mode converter is presented. The TE10 rectangular waveguide mode is converted to thecircular polarization TE11 circular waveguide mode, the pill-box window of the vacuum envelope is used. Employing thethree-dimensional simulation code, the mode converter is designed and optimized. The results show that the VSWR is 1.17 in thecenter frequency of 35 GHz, the mode conversion efficiency is more than 83% in the 34-36 GHz frequency range. This modeconverter possessed of simplicity, compact and easy manufacture can be used in gyro-traveling-wave amplifier tube input coupler.