The design of the high power millimeter wave transmission line for Electron Cyclotron Resonance Heating (ECRH) system on the HT 7 Tokamak was given. The waves output from the gyrotron with peak power of 200kW, frequency 60GHz and pulse length around 100ms was transmitted for several tens of meters through this line. In the transmission process, the wave's mode was converted from initial TE 02 mode to TE 01 and then to the semi linear TE 11 mode by two mode converters. In the end of the transmission line, the wave was coupled to the semi optical antenna and then was launched into plasmas along a special angle.