Abstract: The 0. 55 THz backward-wave oscillator, with slot-structured rectangular grating involving cylindrical electron beam, was developed. Dimensions of the slot structure, including the period length, slot width/depth, rectangular-grating width and waveguide height, were optimized by numerically solving the theoretically derived dispersion equation with Matlab. The beam-wave interaction was simulated with CST MICROWAVE STUDIO software.The simulated results show that the eigen-mode frequency and interaction impedance of the slot-structure are higher than those of the slot-less structure. The length of the backward-wave oscillator was determined by compromising between the oscillation-start time and output power. Besides, the influence of the slot-depth, filling-rate, and current on the output power was investigated via CST and Magic simulations. The optimized slot-depth, filling-rate and current resulted in a steady output power of 14. 6 W. The CST and Magic simulation results were found to be basically in agreement.