X-band dual frequency coaxial relativistic backward wave oscillator is suggested. The properties of dual frequency RBWO are investigated and optimized with electromagnetic simulation method. The results shown that steady dual frequencies microwave power outputs are generated with the operating voltage 590KV, beam current 9.4KA and the external guiding magnetic 0.7T. The microwave frequencies are 9.367GHz and 9.717GHz respectively with frequency differences 350MHz. The averaged output power is 0.75GW with efficiency 13.5%. The power amplitude difference is about 0.14dB.