The ultra-rough surface of the 3D-printed metal produced by laser powder bed fusion technique is one of the major challenges faced in commercializing 3D-printed metallic components in a wider context. The electrochemical polishing method is currently considered a promising method to overcome this obstacle. However, evaluating the average roughness of a 3D printed surface before and after polishing with the current state-of-the-art laser scanning microscopy suffers incomplete datasets due to highly scattered signals. Here we successfully evaluate the rms roughness of the metallic surfaces with different roughness levels using terahertz time-domain spectroscopy (THz-TDS) for comparison against roughness values acquired using a commercial laser scanning microscope.