The dielectric-loaded rectangular waveguide grating traveling wave tube (TWT) is a new kind of high power millimeter wave device. The “hot" dispersion equation of this circuit loaded with an electron beam is presented in this paper， and the “Newton downhill" method is used to solve the transcendental equation. The investigations of the effect of dielectric-layer dimensions and electron beam parameters on the small signal gain are carried out. The calculation results indicated that dielectric-loading can widen the bandwidth of the rectangular waveguide grating TWT and decrease the gain. In order to make up the gain-decrement，the groove-depth can be slightly increased，or the current of the electron beam enhanced within certain limits. The presented analysis will be a guide to the design of this kind of TWTs.