Abstract: A quasi-optical（QO)mode converter with smooth wall launcher used for whispering gallery mode gyrotron oscillators at 95 GHz is designed.This QO mode converter transforms the high order modes in gyrotron into Gaussianlike beams efficiently.A highly efficient simulation code is developed with the method of Stratton-Chu formula and physical optics.With this code the nearfield radiation of every component is calculated，optimized and analyzed.Specific design parameters about the launcher and mirrors for interest modes（e.g.TE62)are obtained.The calculation results show that at the output window， the fundamental Gaussian mode content（FGMC)is 98% and the conversion efficiency of Gaussian mode is about 81.5%.