Abstract: On a certain approximation, the generation and the oscillation of microwave in the cavity based on transit-time effect for one shot microwave source are analyzed. For reasons given, we have made theoretical design and numerical simulation of radial klystron oscillator for GW level output. A peak value (average over a period) of 1.6 2GW ( f = 6.0GHz) is obtained. The width of the microwave putse is 40ns under the condition of the potential pulse width of 116ns. An electron efficiency is about 12.8%.