Abstract-A research of a quasi-optical Bessel-Gauss resonator (QOBGR) at millimeter (MM) and submillimeter (SubMM) wavebands is presented in this paper. The design is based on the quasioptical theory and technique. The iterative Stratton-Chu formula (ISCF) algorithm is employed to analyze the output characteristics of the cavity, including the resonant modes, phases, power losses and phase shifts. Analysis of the results demonstrates that the present design of the QOBGR can support zero order or any high order mode of the pseudo Bessel-Gauss beam. At the output plane the intensity distributions of these modes are modulated by a Gauss-shaped envelope, and their phase patterns have an approximate block-like profile. Tolerance analysis for the designed QOBGR is also done. Lastly, a comparison of resonating modes is made between QOBR (quasi-optical Bessel resonator) and QOBGR when both are configured with the same geometric parameters.