Abstract: In this paper the coupling impedance of circular comb slow wave structure is investigated in detail. Some problems in the computer simulation for interaction impedance are investigated, and a new method for the calculation of interaction impedance is proposed. The study leads to the conclusion that the circular comb slow wave structure has an impedance over 1 W at its fundamental mode within a relatively wide band and may interact with the beam effectively. Besides, the inner radius of the structure is 1/4~1/3 times the wavelength, and 5 times larger than that of a helix, implying 25 times of beam current and 25 times of average power compared to a helix. Calculation of interaction shows that a traveling wave tube in Ka band with maximum power of 170 kW may be realized using this kind