Abstract: A new beam premodulation structure of the coaxial virtual cathode oscillator is proposed and numerically studied. Radial beam premodulation type coaxial virtual cathode oscillator has a metal ring in beam-wave interaction region forming a resonant cavity which changes the electric field distribution of the beam-wave interaction region and modulates the electron beam. The electric field generated by the buncher both modulates the electron beam, and locks the microwave frequency. The resonant frequency is dominated by the length of the metal cylinder and the radial distance between the two cylinders. Through optimization, applying a 600 kV voltage at 73 kA current without external guiding magnetic field, the generated microwave has an average power of 6.00 GW at 2.575 GHz operating frequency, the conversion efficiency reaches 13.94%.