A millimeter-wave backward wave oscillator using a high order space harmonic operation of a folded waveguide and a low-voltage linear electron beam is disclosed. The oscillator has an electron beam generator generating a linear electron beam, a beam collector, and a folded waveguide. The waveguide is bent to an electric field plane and has a rectangular section, and a beam tunnel hole for the electron beam passes therethrough. An electromagnetic wave of millimeter-wave is generated in an opposite direction to the electron beam direction by a high order space harmonic operation of the waveguide, in response to the linear electron beam.