Compared with the conventional helix and coupled cavity traveling wave tube,new kinds of slow wave circuit have many advantages in some aspects.Folded waveguide circuit has played an important role in the millimeter wave and terahertz wave traveling wave tubes,in this paper,the designing method and hot measured results for a series of folded waveguide TWT covering the range from Ka band to 220GHz in our Lab,which have high efficiency and high power characteristics,will be introduced.Meanwhile,some other new sow wave circuit has also been investigated to further improve the performance of TWT,some theoretical and cold measured results will also be given in this paper.