Abstract: A two and one half dimensional electromagnetic relativistic particle-in-cell simulation code has been used to investigate the nonlinear beamwave interaction and other optimization issues of a 3cm relativistic backward wave oscillator in the paper. The simulation results such as microwave generating efficiency, peak power and frequency are reasonable agreement with experiment data. The dependence of the power on electron beam voltage, eletron beam current, slow wave structure geometry and magnetic guide field were simulated, optimum conditions for the device have been obtained.