An analysis of a plasma-filled backward-wave oscillator, in which the axial magnetic field is not employed, is presented using fluid model. The operating modes, output frequencies, and linear instability are, respectively, investigated. The results show that the background plasma affects the output frequency obviously, and that the growth rates are closely related to the electron beam radius, modes of the rippled-wall waveguide, and the background plasma density. The growth rate can be increased as long as the beam radius is made larger. In addition, the present investigation also indicates that the results, given by adding an infinitely strong guide magnetic filed in the device, do not completely apply to the case of no guide magnetic field; thus the results obtained in this paper are significant for the backward-wave oscillator in which the background plasma could be used instead of the guide magnetic field to confine the electron beam against the space-charge effects.