Abstract: A structure of a high power microwave oscillator based on the couple between two cavities is proposed and studied. The structure composes of two cavities and a drifting tube. The couple between two cavities is important for the oscillation to occur under the controlled condition. The structure acts more like a klystron than a distributed traveling wave device. As a result, the oscillator has higher efficiency and its working mode is controlled without mode-competition. The large signal physic process is studied based on the self-consistent theory and the interaction factor k is introduced to study interactions between the beam and the structure. Finally, a conceptual design of a S-band bitron driven by a 700 keV, 10 kA electron beam has been designed.