New　micromachining　techniques　now　provide　the　technology　to　fabricate　slow　wave　structure　(SWS)　which　are　commonly　used　in　the　backward　wave　oscillators　(BWOs)　with　dimensions　suitable　for　operation　in　the　terahertz:　region.　In　this　paper,　results　of　theoretical　and　numerical　simulation　studies　of　a　large　cross　section,　cylindrical　SWS　with　the　rectangluarly　corrugated　inner　wall　are　presented.　The　dispersion　relation　of　the　overmoded　SWS　with　various　dimensional　parameters　is　analyzed,　and　the　distribution　of　the　electrical　fields　in　the　specific　SWS　is　also　determined.　It　is　found　that　the　steady-state,　single-mode　operation　in　this　overmoded　situation　could　be　realized　by　the　performance　of　surface　wave　near　the　upper　cutoff　point.　Under　the　condition　of　550kV　beam　voltage　and　1kA　beam　current,　2.5D　particle-in-cell　simulation　predicted　that　the　proposed　SWS　could　be　employed　in　the　BWO　system　to　deliver　an　output　power　above　40　MW　at　the　frequency　of　about　0.144THz　with　perfect　time　plot　and　fine　spectrum　characteristic.