An oversized rectangular corrugated backward wave oscillator is designed and theoretically analyzed to produce TM01 high power microwave radiation in the frequency of 8.68 GHz without mode competition. The device includes a slow wave structures, a resonant reflector, and an output waveguide. The slow wave structure and the reflector are matched to achieve mode selection and enhance interaction efficiency. Feedback processes from the reflector are introduced to explore mode selection in the configuration. In particle in cell simulation, high power microwaves of TM01 mode are generated with peak power of 2.0 GW, frequency of 8.68 GHz, and conversion efficiency is 28.6% when beam voltage is 500 kV and current 14 kA.