Abstract: An X-band dual-frequency coaxial relativistic backward-wave oscillator with a resonant reflector is proposed in this paper. The 2.5-D fully electromagnetic particle-in-cell code simulation results indicate that, with an electron beam of 520 kV and 8.5 kA guided by an axial magnetic field of 2.35 T, an average power of 1.35 GW with power conversion efficiency of 30.5% is obtained and the two dominant frequencies are 9.41 GHz and 10.29 GHz, respectively. Furthermore, the dual-frequency spectrum and the smaller dominant frequency demonstrate a periodic-like dependence on the drift waveguide, and a frequency agility bandwidth of about 400 MHz is acquired.