In this paper, a ridge-loaded folded-waveguide slow-wave structure (SWS) for millimeter-wave traveling-wave tube (TWT) is presented. The theory of high-frequency characteristics, which includes: 1) the dispersion properties; and 2) the beam-wave interaction impedance of this structure, is analyzed. The theoretical results agree well with those obtained by the 3-D electromagnetic simulation software HFSS. The relationships of the dispersion characteristics and the interaction impedance versus ridge dimensions are numerically calculated and discussed. It is indicated from the investigation that the ridge loading can decrease the phase velocity and the relative bandwidth; however, the interaction impedance increases noticeably. A ridge-loaded folded-waveguide SWS with proper ridge dimensions may be applied for higher gain and electron efficiency in millimeter-wave regimes.