Abstract: A particle�incell (PIC) code is used to simulate the physical mechanism and interaction process of the relativistic backward wave oscillator (RBWO) filled with neutral gas. The simulation results reveal the production process of the plasma due to the collision of the relativistic electron beam (REB) with the neutral gas injected previously into the BWO tube. It is well known that the collision produced plasma may efficiently neutralize the space charge effect of the REB and enhance the production rate of the high power microwave. Our simulation results verify the above conclusion. It is shown that the increasing of the neutral gas density makes the output frequency obviously upper shift and the output power and energy conversion efficiency in the RBWO filled with neutral gas are higher