The relative permittivity (real and imaginary component), absorption coefficient, and loss tangent of various cellulose nanocrystal (CNC) films, a dissolving pulp film, and a CNC powder are obtained by performing terahertz (THz) transmission spectroscopy experiments. The CNC films are constructed using different drying techniques (i.e. air-drying and freeze-drying) and are made from CNCs that have been extracted from various sources (i.e. hardwood, softwood, and dissolving pulp). Between frequencies of 0.2 and 1.5 THz, the real component of the permittivity is seen to range from 1.8-3.3 for the CNC films, suggesting that both the drying technique and CNC source material influence this dielectric property. Importantly, the CNC films are shown to exhibit relatively small THz absorption and loss tangent properties, such that CNC-based dielectric mirrors, waveguides, and transistors may be achieved.