A novel miniaturized rectangular slow-wave structure (SWS), composed of both H-plane and E-plane corrugations, is proposed for submillimeter or terahertz vacuum electron traveling-wave tube (TWT) devices. The advantage of this SWS is to enhance the interaction impedance, therefore, resulting in higher gain and improved output power of the device. In addition, this structure geometry permits design flexibility to achieve a better dispersion behavior (linear dispersion and wider bandwidth) and easy fabrication by available microfabrication processes. Incorporating an H-plane and E-plane load in the SWS design, we achieved a higher performance TWT amplifier with the central frequency of 400 GHz. Both electromagnetic characteristics and beam-wave interaction analysis are investigated using the 3-D electromagnetic software Computer Simulation Technology studio. The simulation results show that an enhanced interaction of the SWS is obtained, and the amplifier has wide instantaneous bandwidth of 80 GHz and 19.5-dB small signal gain at 400 GHz for 17-kV beam voltage and 20-mA beam current. A saturated output power of more than 19 W is obtained from the large-signal simulations.