The corrugated rectangular waveguide is particularly suitable for microfabrication of slow-wave structures for vacuum amplifiers in THz frequency range. An analytical model for the corrugated rectangular waveguide has been extended to compute the cold parameters (dispersion and interaction impedance). A validation procedure demonstrates the validity of the model in THz frequency range. A study on the positioning of the electron beam in the slow-wave structure, based on the interaction impedance evaluation has been carried out. Monte Carlo analysis of a corrugated rectangular waveguide traveling wave tube (TWT), performed by the use of the proposed model demonstrates its effectiveness in the design at THz frequencies, without the need of 3-D simulators in this design phase.