Abstract: The staggered double vane was employed as the slow wave circuit and a transition structure and an output coupler were proposed to design a W-band backward wave oscillator (BWO) operating with sheet electron beam. The transition structure and the output coupler can effectively reduce the attenuation and reflection of such type of straight-waveguide device. Compared to conventional pencil beam device, the oscillator can provide much higher output power. The dynamic process between the electron beam and the electromagnetic wave was simulated by three dimensional particle-in-cell (PIC) code. The simulation results demonstrate that the BWO can provide a stable output power in the order of a few watts in the frequency range from 92 GHz to 98 GHz by adjusting the operation voltage, assuming a constant operation current of 12 mA. The output signal shows good monochromaticity, with the central frequency quite close to the design.