In this paper, experimental findings on the coaxial virtual cathode oscillator at Institute of Applied Electronics of CAEP are reported. The experiments show that the output microwave power and spectrum of a coaxial vircator with the reflector are evidently superior to those of a coaxial vircator without the reflector. With 350 kV diode voltage and 23 kA diode current, 500 MW output microwave at 3.3 GHz is obtained, and the conversion efficiency is about 6.2%. The cavity resonance effect on the coaxial virtual cathode oscillator is investigated.