Results　of　theoretical　and　numerical　simulation　studies　of　a　large　cross　section　(overmoded),　surface　wave　oscillator　(SWO)　with　a　cylindrical　slow　wave　structure　(SWS)　are　presented.　The　dispersion　relation　of　the　overmoded　SWS,　which　express　the　phenomenon　of　mode　competition,　is　calculated,　and　the　distribution　of　electrical　fields　in　the　device　is　also　determined.　A　novel　particle　in　cell　simulation　tool　is　developed,　aiming　at　the　design　of　vacuum　electron　tubes　in　the　terahertz　regime.　It　is　found　that　the　steady-state,　single-mode　operation　in　this　overmoded　situation　could　be　realized　by　the　performance　of　surface　wave　at　＂π＂　point.　Under　the　condition　of　550kV　beam　voltage　and　1kA　beam　current,　2.5D　PIC　simulation　predicted　that　the　proposed　SWO　could　deliver　an　output　power　above　40　MW　at　the　frequency　of　about　0.144THz　with　perfect　time　plot　and　fine　spectrum　characteristic.　©　2008　Institute　of　Fluid　Physics,　C.