Abstract: A rhombus-shaped microstrip meander-line slow-wave structure is proposed for use in a low voltage, wide bandwidth, moderate power and high efficiency millimeter-wave traveling-wave tube. The structure is evolved from the original V-shaped microstrip meander-line slow-wave structure. Compared with the conventional slow-wave structure, it is a kind of planar structure, whose fabrication can be easily realized by utilizing the technology of 2-D micro-fabrication. Whats more, the structure can realize beam-wave interaction with sheet electron beam, and no additional electron beam channel is needed. The electromagnetic characteristics and the interaction between the sheet electron beam and slow wave in the structure are obtained by utilizing the electromagnetic simulation software HFSS and the particle-in-cell code in CST Particle Studio, respectively. Our calculations indicate that, at a beam voltage of 7 kV and a beam current of 90 mA, the traveling wave tube based on the structure is capable of delivering several tens of watts output power with an interaction efficiency of 14.3％ and a transient 3 dB bandwidth of 18 GHz(ranging from 132 GHz to 150 GHz)