Abstract: An S-band relativistic backward wave oscillator without external guiding magnetic field has been investigated using the Karat 2.5D particle-in-cell simulation code. An anode foil is used to extract the electron beam and the non-uniform slow-wave-structure is designed for increasing the microwave output power of the device. Detailed beam-wave interaction pictures in the oscillator were presented. The simulation results show that a 2.79 GHz, 158 MW high power microwave can be obtained when the beam energy is 330 kV and the beam current is about 2.83 kA. The oscillator starts to oscillate at 15 ns, and the nonlinear saturation of the device occurs at about 25 ns. The efficiency is about 16.8% after saturation.