A quasi-optical mode converter transforming TE6,2 mode to Gaussian-like mode is proposed to be employed in 94 GHz Gyrotron. A simulation code is written to model electromagnetic behaviour related to the converter based on the geometric optics and vector diffraction theory. Initial geometry parameters of the converter obtained according to the geometric optics are optimized to make the converter high efficient. Optimization gives a Gaussian mode content of more than 70% and an approximate transmission efficiency 85% for TE6,2 mode at 94 GHz.