Abstract: Theoretical investigation on a highly efficient quasi-optical mode converter for a 0.42THz-TE_{17, 4} gyrotron is presented in this paper. The converter consists of a dimpled-wall Denisov-type launcher and three mirrors. Based on the coupled-mode theory, the vector diffraction theory, and phase unwrapping technique, a computer code has been developed to evaluate the field distributions, which agrees well with the simulated result. It shows that a well-focused wave beam is achieved with a high Gaussian mode purity (scalar content greater than 99.1%, vector content greater than 97.3%) and high power conversion efficiency of greater than 97.4% at the output window.