Big-bore and flexible dielectric-coated metallic waveguides were fabricated with various dielectric film thicknesses. Transmission characteristics were measured for the metallic waveguides and dielectric-coated metallic waveguides in G band, 4.3 THz, and mid-infrared regions. Measured results show that waveguide loss in G band increases with the increase of dielectric film thickness. Loss for the 2.6 mm-bore metallic waveguide is 2.1 dB/m at the frequency of 160 GHz and the additional loss caused by bending for the waveguide is small. Waveguide loss at 4.3 THz decreases with the increase of dielectric film thickness. And the additional loss caused by bending for the dielectric-coated metallic waveguide decreases with the increase of dielectric film thickness. Loss for the 3.6 mm-bore dielectric-coated metallic waveguide with 1.2 μm dielectric film thickness is 2.84 dB/m. Measured beam profiles show that energy is more concentrated in the low-order transmission mode as the film thickness increases.