Abstract: When the operating mode wave generated in a gyrotron is going through the output window section, in which the undesired scattering modes that may disturb the normal operating of gryo-tron can be excited by the discontinuities. The transmitting properties of the window section in a gyrotron are analysed theoretically. The approximate formulas for mode scattering on the radial step of cylindrical waveguides are derived. The frequency dependence of wave transmis-sions of the operating mode and undesired modes through the window system are obtained and discussed. The cold measurements show that the theoretical results are reasonable.