The invention discloses a plastic waveguide for guiding terahertz (THz) wave with a wavelength ranging from 30 to 3000 .mu.m. The plastic waveguide includes a core and a cladding layer. At least part of the core is made of a first plastic medium having a first refractive index, and the maximum length of a cross-section of the core is smaller than the wavelength of the guided terahertz wave. The cladding layer surrounds the core and has a second refractive index lower than the first refractive index. In the invention, only one wave mode is propagated in the plastic waveguide, and a first attenuation constant of the core for the guided terahertz wave is higher than a second attenuation constant of the cladding layer for the guided terahertz wave.