The transmission loss of WR2.8 waveguides made of copper, iron-nickel and gilded iron-nickel alloy at 340 GHz is presented in this paper. The measurement shows the process of gilding at iron-nickel alloy surface can reduce the transmission loss. The test results are compared with those of simulations in order to determine the settings for CST simulation of loss of these materials at 340 GHz. This study is of importance for reducing transmission loss of windows at 340 GHz and setting proper loss parameters for CST simulation of material loss.