<p>In this article, the influence of the surface roughness (effective conductivity) on the open cavity of a gyrotron is analyzed and discussed theoretically. Based on electromagnetic field and wave theory, rough surface effects and metallic conductivity are analyzed considering terahertz cylindrical waveguides. The electromagnetic field distribution of a 360° cylindrical waveguide in TE₄₈ mode is divided into eight parts, and every part is TE₁₁ mode with 360/8° = 45° angle. The TE₁₁ mode was designed by analyzing the TE₄₈ mode. The electromagnetic field distribution of TE₁₁ mode on the radial surface of the sector waveguide is as one-eighth (45° angle) of the surface of the cylindrical waveguide in TE₄₈ mode. This cylindrical waveguide’s radius of TE₄₈ mode is 1.375 mm, for which the operating frequency is 1 THz. MATLAB tool is used to analyze the cylindrical waveguide’s field distribution profile and electrical field characteristics. The characteristics of the effective conductivity of different cavity-cutting materials on smooth surfaces and rough surfaces are studied through numerical simulations. The results indicate that the attenuation coefficient (α) decreases with a rise in frequency considering enhancement in the surface roughness and metal conductivity. TE₄₈ mode of the cylindrical waveguide and TE₁₁ mode of the sector waveguide are discussed through MATLAB analysis. By optimizing through CST simulation, better numerical simulation results are obtained. Overall, this article contributes to attaining the design of the 1 THz gyrotron and a clear understanding of surface roughness improvement for the cylindrical waveguides.</p>