A model of the metal-grating periodic slow-wave structure (SWS) with dielectric loaded in the grooves was built and the expressions of the dispersion equation and coupling impedance were developed by using the Borgnis function and the field-matching method. Under the assumption of the dilute electron beam, the gain was obtained. Through numerical calculations, the influences on the high-frequency characteristics of some parameters are given. Moreover, the different characteristics of the two ways of the dielectric being loaded have been analyzed.