The ridge-loaded helical traveling wave tube (TWT) is a new kind of high power millimeter wave device. The “hot” dispersion equation of this circuit loaded with an electronic beam is presented in this paper, and the Downhill method is used to solve the transcendental equation. The physical dimensions of a 474GHz ridge-loaded helical groove slow wave structure (SWS) with a 20kV electron beam are given and the investigations of the effect of the ridge dimensions and electron beam parameters on the small signal gain are carried out. It is indicated from the calculation results that this type of structure is suitable for use as s SWS of high power, high gain but narrow passband millimeter TWT, of which the 3dB gain bandwidth is 34%. In order to increase the bandwidth, the reduction of the ridge gap or the increase of the electron beam current in an allowable range may be useful. The presented analysis will be a guide to the design of this kind of traveling wave tube.