Abstract: The software MWS (microwave studio) which is provided by the CST company in German was introduced to simulate and calculate dispersion relation and coupling impedance. The high frequency characteristics of the inner slotted rectangular waveguide grating was simulated. The results indicate that the slotted rectangular waveguide grating has lower phase-velocity than the un-slotted one. When it is used as the slow-wave structure of the traveling wave tube, the operating voltage decreases, and the coupling impedance also decreases but it lies above 45 Ω, which satisfies the need of millimeter wave traveling wave tube (TWT). Meanwhile, keeping the inner slotted width constant, it is very useful to increase the groove-depth, grating-spacmg and grating-period for improving the phase velocity of