Abstract: A new kind of coaxial slow wave structure for traveling wave tube is developed in this paper, which can be regarded as disk-loaded waveguide with coaxial dielectric rod winded by metal helix. By means of field matching method, the dispersion equation and coupling impedance for that kind of slow wave circuit are obtained. Numerical results show that kind of circuit can work in a broad operating bandwidth, and its coupling impedance can reach 15 ohms or so. The results presented in this paper provides basis on designing this kind of slow wave structure.