Abstract: The structure and working principle of a Ka-band RBWO operating at TM₀₂ mode with low guiding magnetic field was introduced, and mechanism of the TM₀₂ mode selection was analyzed. The PIC (particle-in-cell) simulation shows the output power and frequency of the RBWO were 493 MW and 29.3 GHz respectively with guiding magnetic field of 1 T. The operating mode and frequency were corresponding to the theoretic design. The experiment study was done based on the parameters in simulation. A 29.3 GHz millimeter wave with an output power of 286 MW and pulse width of 10 ns was obtained at guiding magnetic field, diode voltage and current of 1 T, 580 kV and 3.56 kA respectively. The frequency of microwave in experiment was in accordance with that in simulation. However, the output power in experiment was much lower, and the pulse width was shortened at post end evidently, which was considered as a result of electrons bombarding at the post cascaded resonators.