Some experiments at Institute of Applied Electronics of CAEP showed that the interaction between the virtual cathode oscillator and the cavity is a key factor in determining the output microwave power and propagating modes. Particularly, we observed that the E-beam plays an important role in the cavity formation. The experiments have been carried out with 350 kV diode voltage and 23 kA diode current, where pulsed microwaves of 500 MW in peak power and 3.3 GHz in frequency have been obtained. The energy conversion efficiency from the electron beam to microwave is about 6.2%, and the output microwave modes are mainly the TM01 mode and TE11 mode. The cavity resonance effect on a coaxial virtual cathode oscillator has been investigated in detail.