We have investigated numerically and experimentallycharacteristics of diode perveance and output microwave power incoaxial virtual cathode oscillator (vircator). In this experiment, the diode perveance characteristics could be described using thediode voltages and currents. The sheath plasma expansion speed isto be about 7cm/μs under A-K gap distance 4 mm. We have alsoinvestigated an influence of anode-cathode (A-K) gap distance onoutput characteristics of high power microwave from coaxialvircator. In this experiment, the maximum microwave power isobserved to be 244 MW, whose efficiency is about 2.75 %, at theA-K gap distance of 4 mm. The microwave frequencies are alsomeasured to be 5.1 ≤ f ≥ 5.9 GHz from combination ofcoupled line band pass filter with 1 GHz band gap and dispersivedelay line. With microwave reflector, the maximum output microwavepower is observed to be about 400 MW, whose efficiency is about7.1 %, at the A-K gap distance 8 mm and microwave reflectorinner-hole diameter of 7.6 cm. The microwave frequencies are alsomeasured to be 4.8 ≤ f ≥ 5.2 GHz.