Operation mode of an oversized backward wave oscillator is studied experimentally, using an oversized slow-wave structure. The dispersion characteristics of the slow-wave structure are determined by the average radius, corrugation amplitude and pitch length of the periodic waveguide. The corrugation amplitude and the corrugation period mainly determine the upper cutoff frequency of the lowest mode. The lower cutoff frequency is determined by the average radius. In this work, a Q-band periodic slow-wave structure is fabricated from aluminum. The diameter of the slow-wave structure is about 4 times free space wavelength. The beam voltage is weakly relativistic, around 50 kV, and the beam current is in the range of 100 A. The output powers of several 10 kW are obtained in the Q-band, i.e., the estimated frequency is in the range of 34–41 GH. The output modes are examined by measuring the radiation patterns. Nonaxisymmetric as well as axisymmetric modes are observed. It is demonstrated for the first time that the nonaxisymmetric and axisymmetric operations of the oversized BWO can be controlled by changing the corrugation number of slow-wave structure. © 2004, The Institute of Electrical Engineers of Japan. All rights reserved.