We are presenting a novel miniaturized rectangular Slow-Wave Circuit for sub-millimetre or terahertz vacuum electron traveling wave tube devices. In order to increase the gain of traveling wave tube amplifier at moderate bandwidth, an H-plane metal load is incorporated in addition to an E-plane metal load in a rectangular waveguide. This proposed circuit has been investigated at 400 GHz central frequency using CST studio suite for both the cold circuit parameters and the beam-wave interaction analysis.