Abstract: The structure and the measuring arrangements of vircator with a large plane velvet cathode diode to induce long pulsed microwave radiation are given. The cathode –anode plasma closed velosity, the maximum pulsewidth of the microwave radiation, the microwave peak power and frequence are measured. It is shown that there exists physically restriction in this kind of flat diode vircator between radiation pulse width, radiation frequence and radiation power. It is pointed too that coaxial vircator will be a new way to get rid of such a restriction.