Abstract: The paper introduces a virtual cathode oscillator (vircator) with quasioptical resonator. The resonator offers selfconsistent feedback and modechoosing mechanisms. It serves the purpose of enhancing the transform efficiency, shortening the saturated time and achieving both stable frequency and narrow bandwidth. The property of the system is analyzed by means of the scalar wave theory. Results of 2.5D numerical simulation of the device are presented. A powerful microwave with quasiGauss mode which is apt to focus and radiate is obtained at 7.0GHz, the time average output power overtakes 1.3GW, and the average efficiency approximates to 10%. The simulation confirms the conclusion of the theoretic analysis.