Abstract: The transition waveguides between folded groove guide slow-wave structure and 3 mm standard rectangular waveguide are designed, which are formed by tapered double groove-loaded rectangular waveguides. The influences of structural parameters on transmission performance are discussed based on the simulation results by CST Microwave Studio. Moreover, the transmission performances and loss characteristics of three transition waveguides, i. e. linearly tapered , parabolically tapered and exponentially tapereddouble groove-loaded rectangular waveguidesare analyzed in W band. The simulation results show that the bandwidth of the exponentially tapered transition waveguide is wider than those of the other two structures, while the voltage standing wave ratio (VSWR) is less than 1.15. Furthermore, the whole length of it is much shorter than those of the other two structures when the VSWR is less than 1.25 within 90-99 GHz. However, the loss of the linearly tapered structure is smaller than that of the other structures in 90-97 GHz frequency range.