Abstract: Experiments on planar vircator were carried out on FLASH Ⅱ accelerator to find its frequency characteristics of the vircator. The results of the experiments were given in this paper. Multi frequencies were observed on planar vircator and they varied with the time in one pulse duration. The frequencies mainly cover C-band and X-band with the main frequency in C-band. The main frequency was produced by the oscillation of the virtual cathode and approximately equaled to the relativistic plasma frequency on it. According to experimental results, the main frequency of planar vircator was independent of the cathode area and anode material, and slightly depended upon the gap voltage, and was in inverse ratio with diode gap.