The backward wave oscillator is a promising and powerful source at THz frequencies. The rectangular corrugated waveguide is an effective solution as slow wave structure to design backward-wave oscillators (BWOs), suitable to be fabricated by photolithographic high-aspect ratio processes. However, assembling and vacuum pumping are a critical issue. In this paper, a corrugated waveguide with the width of the corrugation narrower than the waveguide width will be investigated as slow wave structure for BWOs. A relevant improvement from the point of view of the assembling, together with even better performance will be demonstrated. Two backward wave oscillators, at 1 THz central frequency, designed with conventional and narrow corrugated waveguide will be compared in terms of output power and frequency band of tuning.