Single-walled carbon nanotube (SWNT) thin films with thicknesses on the order of 100's nanometers on substrates and graphene thin films on substrate are characterized via Terahertz time-domain spectroscopy. The thin film is treated as a surface boundary condition between the substrate and air. Using the uniform field approximation, the surface conductivities of these films are extracted. The SWNT results show consistent surface conductivities for samples on different substrates and with different film thicknesses. The measured graphene Terahertz conductivity is comparable to the values reported in literatures at DC and optical frequency. This characterization method has been successfully applied as a means to evaluate metallic content of SWNT samples to verify a metallic SWNT purification process using high power microwave irradiation.