Abstract: The folded waveguide is widely studied in the world due to its broad band. The transition waveguides between folded waveguide slow-wave structure and 8 mm waveguide are studied by using numerical method. Three transition waveguides, i.e. linearly tapered waveguide, dualarc waveguide and Chebyshev stepped waveguide, are designed for millimeter wave folded waveguide traveling wave tubes, and their merits and demerits are analyzed. The relationship between the dimensions and performance of the waveguides is obtained. The simulation results show that the reflectivity is below 0.05 within 28~40 GHz for the three structures with rational optimization．