According to the requirements of 140 GHz gyrotron oscillator in thermonuclear fusion, the study for Gaussian mode output windows is conducted. Chemical Vapour Deposition(CVD) diamond is chosen as the output window material, the size of low reflective and absorption window is designed by theoretical analysis, a radius and thickness of the diamond window are 46 mm and 1.8 mm, respectively. In this paper, the influence on the transmission of Gaussian mode and else parameters are analyzed by using theoretical analysis and FEKO software; finally, the design parameters of Gaussian mode output window are obtained. This work has laid a technical foundation in the application of the gyrotron for thermonuclear fusion.