Abstract: Considering azimuthal space harmonics, the finite size vane model is suggested for vane-loaded helical slow-wave structures in this paper. Two vane-loaded helical slow-wave structures of TWT are calculated by the derived dispersion equations. The calculating results are compared with the results of measurement and the calculating results of simple vane model. The influence on dispersion characteristics is analyzed for different center angle of the vanes, radius of the tip of the vanes and radius of metal envelope.