Abstract: Unmagnetized plasma filled relativistic backward wave oscillator RBWO is analyzed by particleincell(PIC) simulation in this paper. The results show that output power is obtained only in a narrow plasma density and has a maximum value. Interaction processes among electron beam, plasma and wave are observed. The electron beam can be transported in the unmagnetized plasma for two reasons. The first, the plasma neutralized the majority of the space charge forces within the beam. The second, radial electric field generated by the electron beam interaction with the plasma leads the beam to conveqdrge along axis. As the background ion effects on the characteristics of the plasmafilled high power devices, inert gas with a high atomic number or mixed inert gases should be used in the devices?