An efficient high power relativistic backward wave oscillator (RBWO) is designed. To enhance the beam-wave conversion efficiency and output power, an overmoded slow wave structure (SWS) is used and an inner conductor is inserted into the SWS. When electron energy is 1.3 MeV while the beam current is 17 kA, the simulated output microwave power is 10.4 GW at 8.8 GHz frequency for the RBWO.